30 nguyenthinhung thpt chuyenhungyen hoa 5191

7/23/2019 30 Nguyenthinhung Thpt Chuyenhungyen Hoa 5191

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CN BNG AXITBAZ TRONG DUNG DCH CHT IN LI

NGUYN TH NHUNG - 1 - THPT CHUYN HNG YN

S GIO DC V O TO HNG YN

SNG KIN KINH NGHIM


Ngi vit:Nguyn Th Nhung

n v cng tc: T Ha

Trng THPT chuyn Hng Yn


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PHN I: MU

1. C s khoa hc ca sng kin kinh nghim cn nghin cu

C s l lun

ib phn cc phn ng ha hc hin nay c xem hoc l phn ng axitbaz hocl phn ng oxi ha kh. Phn ng axitbaz l rt quan trng c v mt nghin cu l thuyt

v mt ng dng ha hc vo thc tin. Trong 10 ha cht m th gii sn xut hin nay th c ti

6 cht l axit hoc baz , l: H2SO4, CaO, NH3, NaOH, H3PO4v HNO3.Phn ng axitbaz l phn ng quan trng, thng xuyn xy ra trong dung dch.

hiu v suy on c tnh cht axitbaz trong dung dch cn cn nm c thuyt Bronxtet v

axit baz.

gii bi ton nh lng cn bit cch m t cn bng trong cc h axitbaz khc

nhau: axit , baz mnh, n axit, n baz yu; a axit, a baz, cc cht lng tnh, cc phchiroxo ca cc ion kim loi. Vic tnh ton thng c tnh theo nh lut tc dng khi lng

v nh lut bo ton proton.

C s thc tinHa hc phn tch ni chung v phn ng axt-baz ni ring c vai tr to ln, chim mt

v tr quan trng trong qu trnh ging dy mn ho hc trng trung hc ph thng, c bit

i vi cc trng chuyn v luyn thi hc sinh gii Quc gia, Quc t. Trong nhng nm qua,

thi hc sinh gii Quc gia thng hay cp ti phn ng axit bazdi nhiu gc khc

nhau. Tuy nhin, trong sch gio khoa ph thng , do iu kin gii hn v thi gian nn nhng

kin thc trn ch c cp n mt cch s lc. Qua thc tin ging dy i tuyn hc sinh

gii Quc gia ti nghin cu, la chn v h thng nhng kin thc l thuyt c bn, trng tm;

su tm nhng bi tp in hnh son chuyn Cn bng axit baz trong dung dchcht

in livi mc ch gip cho hc sinh hiu su v vn dng c tt nhng kin thc hc

vo vic gii cc bi tp, gp phn nng cao cht lng ging dy v hc tp mn Ha hc.

2. Mc ch ca sng kin kinh nghimXy dng h thng l thuyt v bi tp v phn ng axit baz trong dung dch cht

in lidng cho hc sinh lp chuyn Ha hc bc THPT gip hc sinh hc tt hn v chun b

tt hn cho cc k thi hc sinh gii ha hc v c l thuyt bi tpphng php gii, gp phn

nng cao cht lng ging dy v hc tp mn Ha hc.

3. i tng nghin cu, phm vi nghin cu ti Cn bngaxit baz trong dung dch cht in li tp trung h thng l thuyt

v su tm cc bi tp in hnh c lin quan n:

A- C S L THUYTI. Mt s khi nim

1. Khi nim axit - baz

2. Cp axitbaz lin hp

3. Ch s hot ion hiro( pH)

4. Hng s phn li axit hng s phn li baz

5. Phn s nng ()

II. Mt s nh lut bo ton

1. nh lut bo ton vt cht

2. nh lut tc dng khi lng(LTDKL)3. nh lut bo ton proton( iu kin proton)


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II. Nguyn tc chung nh gi thnh phn cn bng trong dung dch

1. Nguyn tc chung v tnh thnh phn cn bng trong dung dch

2. Tnh gn ng khi h ch c mt cn bng ch yu

III. Dung dch cc axit - baz

1. Axit mnh v baz mnh

2. n axit yu v n baz yu

3.Hn hp cc n axit v n baz4. a axit v a baz

5.Cc cht lng tnh

6. Dung dch m

B. CC DNG BI TON MINH HA PHN NG AXIT-BAZ TRONG DUNG DCH

I. Cn bng trong dung dch axit mnh v baz mnh

II.Cn bng trong dung dch n axit yu hoc n baz yu

III. Cn bng trong dung dch cha hn hp axit mnh v n axit yu hoc baz

mnh v n baz yu

IV. Hn hp cc n axit yu v hn hp cc n baz yu

V. Cn bng trong dung dch a axit v a baz

VI. Cn bng trong dung dchcc cht lng tnhi tng nghin cu l cc kha hc sinh i tuyn d thi hc sinh gii quc gia t nm

2011 n nm 2013

4. K hoch nghin cuSng kin kinh nghim ny ti nghin cu ging dybi dng hc sinh i tuyn d thi

hc sinh gii quc gia, ti trngTHPT chuyn Hng Yn t nm hc 2011-2013.

5. Phng php nghin cua) Nghin cu ti liu.

b) Thc nghim (ging dy), y l phng php chnh.6. Thi gian hon thnh

2 nm

7. Cu trc ca sng kin kinh nghim

Ngoi phn m u,kt lun v ti liu tham kho, sng kin kinh nghim bao gm cc phn

chnh sau y:

A. C S L THUYT

B. CC DNG BI TP MINH HA PHN NG AXITBAZ TRONG DUNG DCH

PHN II: NI DUNG

A- C S L THUYT

I. Mt s khi nim

1. Khi nim axit - baz

Ti liu gio khoa chuyn ho hc a ra 3 nh ngha v axit-baz nh sau:

*Theo Areniut( Arrhenius):

- Axit l nhng cht c kh nng phn li trong dung dch thnh cation hiro(H+), cn baz l

nhng cht c kh nng phn li trong dung dch thnh anion hiroxit( OH-).

*Theo Bronstet v Lauri:


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- Axit l nhng cht c kh nng nhng proton(H+), cn baz l nhng cht c kh nng nhn

proton(H+).

Cc cht lng tnh va c tnh axit( cho proton), va c tnh baz( nhn proton).

*Theo Liuyt:

- Axit l nhng cht c kh nng nhn cp electron khng phn chia, cn baz l nhng cht c

kh nng cho cp electron khng phn chia.

2. Cp axitbaz lin hpMt axit khi cho proton th bin thnh baz tng ng v mt baz khi nhn proton th bin

thnh mt axit tng ng, ngha l bin i thun nghch:

HA( axit) A-baz) + H+.Trong qu trnh thun nghch ny HA v A-hp thnh mt cp axitbaz lin hp. i vi cp

axit-baz lin hp ta lun c: Ka.Kb= Kw.

3. Ch s hot ion hiro( pH)

pHc trng cho tnh axit-baz ca dung dch.

Biu thc: pH= -lg(H+) hoc pH= -lg[H+] (i vi cc dung dch long).

Nu dung dch c [H+] = 10-a pH = a

Trong dung dch axit: [H+] > 1,00.10-7M; pH < 7,00; pOH > 7,00

Trong dung dch baz : [H+] < 1,00.10-7M; pH >7,00; pOH < 7,00

Trong mi trng trung tnh: [H+] = [OH-] = 1,00.10-7M; pH= 7,00

Trong mi trng nc lun lun c biu thc: Kw= [H+].[OH-] = 10-14.

4. Hng s phn li axit hng s phn li baz

nh gi mnh yu ca axit-baz phi da vo hng s axit v hng s baz:

Hng s axit Kac tnh theo cng thc: HA + H2O H3O++ A-

HAAOH

Ka

.3

i vi cc a axit th mi nc phn li c hng s c trng cho qu trnh :

H2A H++ HA-

AHAH

Ka2

.1

HA- H++ A2-

HA

AHKa

2.2

Cn hng s baz ca cc baz yu cng c tnh da theo cn bng ca baz, v d: NH 3+

H2O NH4++ OH-

34 .

NH

OHNHKb

5. Phn s nng ()5.1. Phn s nng ca n axit, n baz

Trong dung dch axit, baz phn s nng il t s gia nng cn bng ca cu t i vi

tng nng cc dng c mt ca i trong dung dch.

i vi cn bng: HA H++ A- Ka

Ta c:

HAHA C

HA

AHA

HA

(5)

Hay HAHACHA . (6)

Tng t: AHACA . (7) . D nhin: 1 AHA (8)


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y :

HK

H

a

HA ;

HK

K

a

aA

(9)

5.2. Phn s nng ca a axit a baz

- i vi axit H2A:

2

2

21 1 2

H Aa a a

h

h hK K K

(IV.1)

12

1 1 2

.

.a

HAa a a

K h

h h K K K

(IV.2)

21 2

21 1 2

.

.a a

Aa a a

K K

h h K K K

(IV.3)

- i vi axit HnA:

1 21 1 2

1

...n

n

H A nn n n

a a a aii

h

h K h K K h K

(IV.4)

1

1 21 1 2

1

...n

n

aii

nAn n n

a a a aii

K

h K h K K h K

(IV.5)

Tng qut:

0

0 0

in i

nn i

aii

inH An i

aii i

h K

h K

vi Ka0= 1 (IV.5)

II. Mt s nh lut bo ton

1. nh lut bo ton vt cht1.1. Quy c biu din nng

* Nng gc: nng cht trc khi a vo hn hp phn ng(C0 mol/l)

* Nng ban u:nng cht trong hn hp trc khi xy ra phn ng

(C0mol/l)

* Nng cn bng: nng cht sau khi h t ti trng thi cn bng([i]).

* Nng mol: biu din s mol cht trong 1 lt dung dch hoc s mmol trong 1ml dung dch(C

mol/l)

* Nng %:biu din s gam cht tan trong 100 gam dung dch.

1.2. nh lut bo ton nng (LBTN) ban uNng ban u ca mt cu t bng tng nng cn bng ca cc dng tn ti ca cu t

khi cn bng: Ci=[i]

1.3. nh lut bo ton in tch(LBTT)[i]Zi=0

Zil in tch (m hoc dng) ca cu t i c nng cn bng [i].

2. nh lut tc dng khi lng(LTDKL)

* i vi cn bng: aA +bB cC + dD )(aK


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ba

dc

a BA

DCK

).()(

).()()(

(i): l hot ca cht i ;K(a)l hng s cn bng nhit ng;

vi (i)=[i].fi; fil h s hot ca i

*i vi cc php tnh gn ng, c th coi cc gi tr h s hot u bng 1 v hng s cn

bng nhit ng K(a)c coi nh l hng s cn bng Kc:

ba

dcc

BA

DCK

.

.

*C th nh gi hng s cn bng ca cc qu trnh phc tp bng cch t hp cc cn bng n

gin bit:

A + B C + D K1

C + D A + B K=K1-1

A + B C + D K1

C + D E + G K2

A + B E + G K=K1K2

nA + nB nC + nD K= (K1)n

3. nh lut bo ton proton( iu kin proton)

nh lut bo ton proton (hay cn gi l iu kin proton (KP)), ni dung ca nh lut

ny nh sau:

Nu ta chn mt trng thi no ca dung dch l chun (thng gi l trng thi quy

chiu hay l mc khng ) th tng nng proton m cc cu t mc khng gii phng ra bng

tng nng proton m cc cu t thu vo t n trng thi cn bng.

Ni cch khc, nng cn bng ca proton c trong dung dch bng hiu gia tng nng

proton gii phng ra v tng nng proton thu vo t mc khng:

[ ] ( ) ( )cho nhani ji j

H H H

( )choi i

H ch tng nng proton giiphng ra.

( )nhanj j

H ch tng nng proton thu vo t cc cu t ca mc khng.

i,j ch cc cu t c kh nng cho v nhn proton.

Trng thi quy chiu (mc khng) c th l trng thi ban u, trng thi gii hn hay mt

trng thi tu chn. Thng thng tin cho vic tnh gn ng, ngi ta thng chn trng thi

trong nng ca cc cu t chim u th lm mc khng.

II. Nguyn tc chung nh gi thnh phn cn bng trong dung dch

1. Nguyn tc chung v tnh thnh phn cn bng trongdung dch

- Bc 1: M t y cc cn bng c th xy ra.

- Bc 2: nh gi, so snh cc cn bng

+ Nu h ch c mt cn bng quyt nh thnh phn cn bng ca h: Ta p dng nh lut

bo ton khi lng cho cn bng t xc nh thnh phn cnbng ca h.

+ Nu h c nhiu cn bng tng ng quyt nh thnh phn cn bng ca h : Ta p dng

cc nh lut khc nhau (LTDKL, LBTN, LBTT, KP...) thnh lp h phng trnh

hoc phng trnh bc cao.


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- Bc 3: Gii h phng trnh hoc phng trnh bc cao.

- Bc 4: Kim tra cc gi thit

- Bc 5: nh gi cc thnh phn khc ca h

2. Tnh gn ng khi h ch c mt cn bng ch yu- Trong trng hp n gin thng gp khi h ch c 1 cn bng duy nht th c th t hp

LBTN vi LTDKL tnh nng cn bng ca cc cu t. Chng hn, trong dung dch

MX C mol/l ch c cn bng.MX Mn++ Xn- K (1).

Theo LBTN i vi MX ta c : CMX= [Mn+] + [MX] = C (2)

Hay [MX] = C - [Mn+] (3)

Theo biu thc LTDKL p dng cho (1) ta c : KMX

XM nn

][

]].[[ (4)

t to phn ng bng x ta c : [Xn-] = [Mn+] = x (5)

Thay (3) v (5) vo (4) ta c : K =xC

x

2

(6)

Gii phng trnh (6) s tnh c x v t phng trnh (3), (5) tnh c nng cn bngca cc phn t trong dung dch.

cho tin, ta ghi s tnh nh sau:

MX Mn+ + Xn- K

Nng ban u: C0 C

C -x x x

Nng cn bng: [ ] C-x x x

x l to phn ng.

Theo LTDKL : K =

xC

x

2

Nu x C ( khi K rt b) thC

x2= K x = CK.

- Trong trng hp K rt ln ngha l phn ng xy ra rt mnh theo chiu thun th cn xc

nh TPGH v tnh theo cn bng ngc vi cn bng cho.

III. Dung dch cc axit - baz

1. Axit mnh v baz mnh

Axit mnh(k hiu l HY) l nhng cht trong dung dch nc c kh nng nhng hon ton

proton cho nc:

HY + H2O H3O++ Y-

Cn bng trn thng c vit di dng n gin : HY H++ Y-Trong dung dch baz mnh(k hiuXOH) ton b lng baz c th thu proton ca nc: XOH

+ H2O X+(H2O) + OH

-

Mt cch n gin c th biu din: XOH X++ OH-

Khi tnh ton c th coi h s hot ca cc cu t u bng 1, do :

pH= -lg(H+) - lg[H+]

2. n axit yu v n baz yu

*n axit yu HA l nhng cht trong dung dch c kh nng nhng mt phn proton cho nc

v dung dch c phn ng axit. mnh ca cc n axit yu c c trng bi hng s phn li


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axit Kahoc ch s hng s phn li axit pKa= -lgKa. Gi tr Kacng ln hay pKacng b th axit

cng mnh.

* mnh yu ca axit yu cng c phn nh qua in li . in li l t s gia s mol

n ca cht phn li thnh ion vi tng s mol n0 ca cht tan trong dung dch. Hoc l t s

gia nng cht phn li vi tng nng cht in li. in li t l thun vi hng s phn

li K v t l nghch vi nng . i vi n axit HA ta c lin h:

CKa

1

2

Trong dung dch axit HA xy ra cc qu trnh sau:

HA H++ A- Ka (1)

H2O H++ OH- Kw (2)

Nu Ka.CHA>> Kwth c th tnh thnh phn ca h theo cn bng(1)

Nu Ka.CHA Kwth phi k n s phn li ca nc.

*n baz yu A-l nhng cht m trong dung dch , mt phn ca chng c kh nng thu proton

ca nc v dung dch c phn ng baz. mnh ca cc baz ph thuc vo hng s baz Kb=

Kw/Kahoc ch s hng s baz pKb= -lgKb= pKw- pKa( vi Ka l hng s phn li ca axit lin hp). Khi Kbcng ln hoc pKbcng b th baz cng

mnh. Nh vy khi axit lin hp cng mnh th baz cng yu v ngc li.

Trong dung dch n baz A-c cc cn bng:

H2O H++ OH- Kw (3)

A-+ H2O HA + OH- Kb=Kw/Ka(4)

Tng t, nu wAb KCK . th c th tnh cn bng theo (4). Trng hp nu

wAb KCK . th phi tnh n qu trnh phn li ca nc.

3.Hn hp cc n axit v n baz

c1)Hn hp cc axit mnh HY C1mol/l v cc axit yu HX nng C2mol/lTrong dung dch c cc qu trnh:

HY H++ Y- (10)

HX H++ X-Ka (11)

H2O H++ OH- Kw (12)

Do s c mt ca axit mnh HY, trong a s trng hp dung dch c phn ng axit nn b qua

s phn li ca nc v thnh phn ca dung dch c tnh theo LTDKL cho cn bng (11),

trong c k n s c mt ca nng ion H+do axit mnh phn li ra.

HX H+ + X- Ka

C C2 C1[ ] C2-x C1+x x

Ta c

xC

xxCKa

2

1 . (13)

c2) Hn hp baz mnh XOH C1mol/l v cc baz yu A-nng C2mol/l

Trong h xy ra cc qu trnh sau:

XOH X++ OH- (14)

H2O H++ OH- Kw (15)

A-+ H2O HA + OH- Kw.Ka

-1 (16)


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Tng t nh trn do s c mt ca baz mnh XOH nn c th coi qu trnh phn li ca nc l

khng ng k, do vic tnh ton cn bng da vo cn bng (16) c k n nng ca ion

OH-C1mol/l do XOH phn li ra: A- + H2O HA + OH

-

Kw.Ka-1

C C2 C1

[ ] C2-x x C1+x

Ta c xC

xxCKK aw

2

11 .. (17)

c3) Hn hp cc n axit HA1(C1mol/l, Ka1), HA2(C2mol/l, Ka2)

Cc cn bng xy ra :

HA1 H++ A1

- Ka1 (18)

HA2 H++ A2

- Ka2 (19)

H2O H++ OH- Kw (20)

Nu waa KCKCK 21 .. 21 th thnh phn cn bng ca h c th c tnh theo(18) nh i

vi dung dch cha n axit HA1.

Trong trng hp cc cn bngxy ra tng ng (tc l waa KCKCK 21 .. 21 ) th tin hnh

tnh lp gn ng theo KP:

21 AAOHHh (21)Sau khi t hp cn thit ta c:

waa KHAKHAKh 21 21 (22)

tnh gn ng bc 1(gi tr h1) chp nhn [HAi]0= Cithay h1vo(23) tnh li [HAi]i:

hK

hCCHA

iaiiii

.. (23)

thay cc gi tr [HAi]1va tnh c vo (22) tnh gn ng bc 2( gi tr h2) v tip tc tnhlp cho n hi t.

c4) Hn hp cc n baz 1

,/11 bKlmolCA v

2,/22 bKlmolCA

Cc qu trnh:

H2O H++ OH- Kw (24)

A1-+ H2O HA1+ OH

-1b

K (25)

A2-+ H2O HA2+ OH

-2b

K (26)

Vic tnh ton cn bng i vi hn hp cc nbaz tng t cc n axit:

Nu wbb KKCKC 21 .. 21 th c th tnh ton theo cn bng (25).Nu wbb KKCKC 21 .. 21 th phi tnh lp theo KP:

21 HAHAOHHh

hay

2

11

1 ..121

AKAK

Kh

aa

w (27)


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tnh gn ng c th chp nhn ii CA v sau kim tra li iA theo biu thc:

hK

KCCA

i

i

a

a

iiii .. (28)

c5) Hn hp cc axit yu HA( Camol/l) v baz lin hp A-( Cbmol/l).

Trong dung dch ngoi cn bng phn li ca nc cn c cn bng:

HA H

+

+ A

-

Ka (29)A-+ H2O HA + OH- Kb (30)

Nu gi tr

710..

b

aaa C

CK

A

HAKH (dung dch c phn ng axit) th vic nh gi thnh

phn cn bng ca h tnh theo(29).

Nu 710. b

aa C

CK (mi trng baz) th tnh ton cn bng da vo (30)

Trong trng hp 710. b

aa C

CK th phi tnh c s phn li ca nc v tnh theo KP:

bCAOHHh hoc aCHAOHh (31)Trong trng hp Ca, Cb>>h, Kw/h v Ka, Kbkhng qu b th c th tnh gn ngpH ca hn

hp n axit(Ca) v baz lin hp(Cb) (dung dch kho st ny c gi l dung dch m):

a

ba C

CpKpH lg (32)

4. a axit v a baz

4.1. a axit:(HnA)Trong dung dch, a axit HnA c kh nng phn li theo tng nc

HnA H++ Hn-1A

-1a

K (33)

Hn-1A- H++ Hn-2A

2-2a

K (34)

....... ..... ........ ......

HA(n-1) H++ An-na

K (35)

C th coi a axit nh mt hn hp gm nhiu n axit.

* Nunaaaa

KKKK .....321

th c th coi s phn li ca a axit xy ra ch yu nc

1 v c th tnh cn bng theo LTDKL p dng cho nc phn li th nht ca axit:

HnA H++ Hn-1A

-1a

K

[ ] C-x x x

xC

xKa

2

1 [H+] =x, t tnh c nng cc cu t khc

4.2.a bazTrong dung dch, a baz c kh nng thu proton tng nc ca nc. Qu trnh proton ho ca a

baz xy ra ngc vi qutrnh phn li ca a axit tng ng:

An-+ H2O HA(n-1)-+ OH- 1.

1

nawb

KKK (39)

HAn-1+ H2O H2A(n-2)-+ OH- 1

12.

nawb KKK (40)

..... ..... ..... .....


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Hn-1A-+ H2O HnA + OH

- 11

. awb KKK n (41)

*Nunbbbb

KKKK .....321

, ngha l qu trnh proton ho nc 1(39) chim u th,

khi c th nh gi thnh phn cn bng ca a baz nh mt n baz :

An-+ H2O HA(n-1)-+ OH-

1bK

[ ] C-x x x

Ta c :xC

xKb 2

1 x=[OH-], t nh gi c thnh phn cn bng ca h.

* Lu : i vi cc a axit v a baz c cc hng s phn li k tip gn nhau th phi p dng

KP tnh lp nng [H+] cho n hi t.

5.Cc cht lng tnh

Cc cht vac tnh axit (cho proton) va c tnh baz (thu proton) c gi l cc cht

lng tnh. Cc cht lng tnh gm cc hidroxit ca 1 s kim loi (nhm, km, crom, thic,

ch) cc aminoaxit, cc mui axit, mui ca axit yu v baz yu.

Xt dung dch mui NaHA, trong dung dch c cc cn bng

NaHA Na

+

+ HA

-

.H2OH++ OH- Kw (42)

HA-H++ A2- Ka2 (43)

HA-+ H+H2A Ka1-1

(44)

Nu chn mc khng l : HA-; H2O.

Ta c biu thc KP: h= [H+] = [OH-] + [A2-] - [H2A] (45)

Sau khi t hp cn thit ta c:

h =].[1

].[1

1

2

HAK

HAKK

a

aw (46)

Trong a s trung hp HA-

phn li yu nn c th coi gn ng[HA-] CNaHA= C, v vy biu thc(46) tr thnh :

h =CK

CKK

a

aw

.1

.1

1

2

(47)

- Nu Kw> 1 hay Ka1


12/36



yu hay baz yu(NH4CN, NH4CH3COO). Bn cht tc dng m c th hin ch: khi thm

vo h mt lng axit mnh th baz lin hp c mt(A-) s thu proton to ra mt lng tng

i b HA so vi lng HA c sn, do pH gim khng nhiu; cn nu thm vo h mt

lng bazmnh th n s b HA trung ho to ra mt lng tng ng baz yu A-, lng

ny tng i b so vi lng baz A- c sn, v vy pH tng ln khng ng k.

i vi h m gm axit yu HA(Ca) v baz lin hp A-(Cb) c cc cn bng:

HA H+

+ A-

KaH2O H

++ OH- Kw

Nu [H+], [OH-]


13/36



Gii:

Bi ton cho bit nng gc ca dung dch, khi phatrn cc dung dch cn phi tm nng

ban u ca cc dung dch(C):

MCHCl 05,0200

1,0.100 ; MCHNO 015,0200

03,0.1003

Cc cn bng xy ra trong dung dch:

HCl H+

+ Cl-

0,05 0,05

HNO3 H++ NO3

-

0,015 0,015

[H+]=[Cl-] + [NO3-] = 0,05+0,015 = 0,065M( y l dung dch axit mnh nn H+do nc

phn li khng ng k).

pH= - lg(0,065) = 1,19

V d 3:Trn 100 ml dung dch HCl 0,10M vi 100ml dung dch NaOH 0,20 M. Tnh pH ca

dung dch thu c.

Gii:y l bi ton trn mt dung dch axit mnhvi mt dung dch baz mnh, v vy ta phi

xc nh TPGH, t xc nh pH ca dung dch.

- Nng gc C0: HCl 0,10 M; NaOH 0,20 M

- Nng ban u C0: HCl = (0,10.100)/ 200 = 0,05M

NaOH = (0,20.100)/ 200 = 0,10 M

Phn ng: HCl + NaOH NaCl + H2O

C0 0,05 0,10

C - 0,05 0,05

TPGH: NaOH 0,05 M; NaCl 0,05 M; H2O

Tnh pH ca dung dch theo NaOH( B qua s phn li ca nc).

NaOH Na++ OH-0,05 0,05

[OH-] = 0,05M [H+] = 10-14/ 0,05 = 2.10-13 pH = -lg(2.10-13) = 12,7

Lu : i vi dung dch hn hp gm axit mnh v mui ca n vi baz mnh hoc hn hp

gm baz mnh v mui ca n vi axit mnh, th khi tnh pH ca dung dch ta tnh theo axit

mnh hoc baz mnh, cn cc mui ca chng khng nh hng g n vic tnh ton pH ca

dung dch.

V d 4:Trn 10,00 ml dung dch hn hp HCl v HNO3 c pH = 2 vi 10,00 ml dung dchBa(OH)2c pH = 12. Tnh pH ca dung dch sau phn ng.

Gii:

Bi ton ny khng cho nng ca cc cht m cho pH ca cc dung dch tham gia phn ng

nn phi da vo pH tm nng mol/l ca cc ion H+v OH-, t vit cc phn ng, ri tm

thnh phn gii hn, thnh phn cn bng v tnh pH.

- Nng gc C0: [H+]= 10-2M

[OH-]= 10-14/ 10-12=10-2M

- Nng ban u C0: [H+]= (10-2.10)/ 20=5.10-3M

[OH-]= (10-2.10)/ 20= 5.10-3M

Phn ng : H+ + OH- H2O

5.10-3 5.10-3


14/36



- -

TPGH: Cl-, NO3-, Ba2+, H2O

Nh vy, trong dung dch sau phn ng ch cha mui ca axit mnh v baz mnh, u l nhng

cht trung tnh nn pH ca dung dch sau phn ng bng 7. y TPGH cng chnh l thnh

phn cn bng.

1.2. Trng hp 2

Nu nng ca axit v baz qu nh )()()()( 22 OHOHbazoOHOHHaxitH ChayCCC th khng b qua cn bng phn li ca H2O trong dung dch, ta phi da vo KP.

V d 5:Thm 9,95 ml dung dch NaOH 2.10-4M vo 10,00 ml dung dch HClO42.10-4M. Tnh

pH ca dung dch thu c.

Gii:

9,95 ml dung dch NaOH 2.10-4M phn ng ht vi 9,95 ml dung dch HClO4 2.10-4M. Vy

HClO4cn d l 0,05 ml MCHClO8

4

101000

10.2.05,04

Vy)(

8

)( 210

OHHH CMC nn khng b qua s phn li ca nc. Cc qu trnh xy ra trong

dung dch:

HClO4 H++ ClO4

-

H2O H++ OH-

KP:

01010 14824 4

HH

H

KCHOHClOH wHClO

[H+]= 1,05.10-7 pH= 6,97

II. Cn bng trong dung dch n axit yu hoc n baz yu

2.1. Dung dch n axit yu.V d 6:Tnh pH ca dung dch axit CH3COOH 1,00.10

-3M. Bit CH3COOH c Ka=10-4,76.

Gii: Cc cn bng xy ra:

CH3COOH CH3COO-+ H+ Ka= 10

-4,76(1)

H2O H++ OH- Kw= 10

-14 (2)

i vi bi ton ny ta phi so snh (Ka.Ca) vi Kw, loi b cn bng phn li ca H2O.

Ta c: Ka.Ca=10-4,76.10-3= 10-7,76>> Kw= 10

-14, vy c th b qua s phn li ca H2O, trong dung

dch cn bng (2) xy ra l ch yu.

CH3COOH CH3COO-+ H+ Ka=10

-4,76

C0 10-3

[ ] 10-3-x x x

xx

3

276,4

1010 x2+ 10-4,76x - 10-7,76= 0 x= 1,23.10-4pH = 3,9

( i vi trng hp ny khng th gii gn ng m phi gii phng trnh bc 2 v gi tr x gn

ng l 1,3.10-4li nh hn 10-3)

V d 7:Tnh nng cn bng ca cc cu t trong dung dch thu c khi ho tan 0,535 gam

NH4Cl trong 200 ml nc. Bit NH4+c Ka=10

-9,24.

Gii : MCmoln MClNH 05,02,0

01,001,0

5,53

535,04

Cc qu trnh xy ra trong dung dch:


15/36



NH4Cl NH4++ Cl-

H2O H++ OH- Kw=10

-14 (1)

NH4+ NH3+ H

+ Ka= 10-9,24 (2)

Ta c: Ka.C = 10-9,24.5.10-2= 5.10-11,24>> 10-14

Vy trong dung dch cn bng (2) xy ra l ch yu:

NH4+ NH3 + H

+ Ka= 10-9,24

C0 0,05[ ] 0,05-x x x

x

x

05,010

224,9 vi x> [OH-]= 10-8,5 c th b qua cn bng phn li ca nc v

tnh theo (2).

Gi s gam NH4Cl cn ly l a Maa

C ClNH 5,53

4

250.5,53

1000.4

NH4+ NH3 + H

+ Ka= 10-9,24

C05,53

4a

[ ]5,53

4a-x x x

24,92

10

5,53

4

xax

trong x= 10-5,5; 24,95,5

10

1010

5,53

410

a a = 2,324 gam

2.2. Dung dch n baz yu

V d 9:Tnh pH ca dung dch NH310-2M.Bit NH4

+c Ka=10-9,24.

Gii:Cc cn bng xy ra trong dung dch:

NH3+ H2O NH4++ OH- Kb= 10

-14/ 10-9,24= 10-4,76 (1)

H2O H++ OH- Kw= 10

-14 (2)

Ta c: Cb.Kb= 10-6,76>> Kw=10

-14 b qua cn bng phn li ca nc. Tnh theo cn bng(1)

NH3+ H2O NH4+ + OH- Kb= 10

-4,76

C0 10-2

[ ] 10-2- x x x


16/36



11476,676,422

276,4 10.46,210.08,401010

1010

HOHxxx

x

x

Vy : pH = 10,6

V d 10:Tnh pH ca dung dch CH3COOK 2,0.10-5 M. Bit CH3COOH c Ka=10

-4,76.

Gii:Cc cn bng xy ra trong dung dch:

CH3COOK CH3COO-

+ K+

2,0.10-5 2,0.10-5

CH3COO-+ H2O CH3COOH + OH

- Kb= 10-9,24(1)

H2O H+ + OH- Kw= 10

-14 (2)

Ta c : Cb.Kb= 2.10-5.10-9,24= 1,15.10-14 Kw=10

-14, nn khng th b qua s phn li ca nc,

khi tnh ton phi da vo KP.

Biu thc KP i vi cn bng(1) v (2) l:

[OH-]= [CH3COOH] + [H+]

aK

COCHH

H

H-

314 O.10

, mt cch gn ng chp nhn [CH3COO

-] C(CH3COO-

)=2.10-5 [H+] = 6,82.10-8 pH = 7,166

* Kim tra kt qu bng cch tnhlp: kt qu hi t ti pH= 7,166

III. Cn bng trong dung dch cha hn hp axit mnh v n axit yu hoc baz mnh v

n baz yu

3.1.Hn hp axit mnh v axit yu

V d 11: Tnh pH ca dung dch thu c khi trn 40 ml dung dch HCl 2,5.10 -2M vi 60ml

dung dch CH3COOH 1,67.10-4M.Bit CH3COOH c Ka=10

-4,76.

Gii:Sau khi trn: CHCl= (2,5.10-2.40)/ 100 = 0,01 M = 10-2M

MC COOHCH44

10100/)60.10.67,1(3


HCl H+ + Cl-

CH3COOH CH3COO- + H+ Ka= 10

-4,76 (1)

H2O H+ + OH- Kw=10

-14 (2)

V Ka >> Kwnn b qua cn bng (2), tnh ton da vo cn bng (1)


-4,76

C0 10-4 10-2

[ ] 10-4-x x 10-2+x

p dng LTDKL ta c:xxx

4

276,4

10)10(10

776,876,422 10.73,1010)1010( xxx

[H+] 10-2 pH = 2

V d 12:Tnh pH ca dung dch thu c khi trn 10,00 ml dung dch H2SO40,01 M vi 40,00ml dung dch HNO3c pH=1,3. Bit HSO4

-c Ka=10-2.

Gii: Dung dch HNO3c pH=1,3 [H+]= 10-1,3=0,05 M = C0(HNO3)

Nng cc cht sau khi pha trn: MC SOH310.2

50

10.01,042

; MCHNO 04,05040.05,0

3


17/36



Trong dung dch c cc cn bng:

H2SO4 H+ + HSO4

-

HNO3 H+ + NO3

-

HSO4- H+ + SO4

2- Ka= 10-2 (1)

H2O H+ + OH- Kw= 10

-14 (2)

Do Kart ln nn c th b qua s phn li ca nc, tnh ton da vo cn bng (1).

HSO4- H+ + SO42- Ka= 10-2C0 2.10-3 0,042

[ ] 2.10-3-x 0,042+x x

p dng LTDKL ta c: 4522 10.82,3010.2052,0002,0

)042,0(10

xxx

x

xx

[H+]= 0,042+3,82.10-4=0,0424 pH = 1,37

3.2. Hn hp baz mnh v baz yuV d 13: Tnh pH ca dung dch gm NaOH 1,0.10-4 M v NaNO20,10 M. Bit HNO2c Ka=

10-3,3.

Gii : Cc qu trnh xy ra trong dung dch:NaNO2 Na+ + NO2

-

NaOH Na+ + OH-

NO2- + H2O HNO2 + OH

- Kb= 10-10,7

C0 0,10 1,0.10-4

[ ] 0,1-x x 1,0.10-4+x

p dng LTDKL ta c:

010)1010.0,1(1,0

)10.0,1(10 7,117,1042

47,10

xxx

xx

x= 1,99.10

-8

[OH

-

]= 10

-4

pH = 10V d 14:Tnh khi lng NaOH phi cho vo 500 ml dung dch HCOONa 0,010 M pH ca

dung dch thu c l 11,50 (b qua s thay i th tch ca dung dch). Bit HCOOH c Ka=10-

3,75.

Gii:

Gis mol NaOH cn thm vo l a mol CNaOH= (a.1000)/ 500= 2a M.


NaOH Na+ + OH-

2a 2a

HCOONa Na+ + HCOO-

0,010 0,010

HCOO- + H2O HCOOH + OH- Kb= 10

-10,25 (1)

H2O H+ + OH- Kw= 10

-14 (2)

V pH= 11,5 [H+]= 10-11,5


18/36



Ta c:x

xax

01,0

)2(10 25,10 trong (2a+x) = 10-2,5

x= 17,8.10-9 a= (10-2,5-17,8.10-9)/2= 1,58.10-3 mNaOH=0,0632 gam

IV. Hn hp cc n axit yu v hn hp cc n baz yu

4.1. Hn hp cc n axit yuV d 15:Tnh nng cn bng ca cc cu t v tnh pH ca dung dch gm CH3COOH 0,010

M(C1) v NH4Cl 0,100 M(C2). Bit CH3COOH c Ka=10-4,75

; NH4+

c Ka=10-9,24

.Gii:

Trong dung dch c cc cn bng sau:

NH4Cl NH4+ + Cl-

0,100 0,100

H2O H+ + OH- Kw= 10

-14 (1)

CH3COOH H+ + CH3COO

- Ka1= 10-4,76 (2)

NH4+ H+ + NH3 Ka2= 10

-9,24 (3)

V: Ka1.C1=10-6,76>> Ka2.C2= 10

-10,24> Kw nn mt cch gn ng ta ch tnh ton cn bng theo

(2)CH3COOH H

+ + CH3COO- Ka1= 10

-4,76

C0 0,010

[ ] 0,010-x x x

COOCHHxx

x3

438,376,42

10.17,4101001,0

pH= 3,38

* Kim tra kt qu:

T cn bng (3) ta c: 38,386,638,3

24,93 1010

10

1,0.10

NH vy s phn li ca NH4

+ l

khng ng k so vi CH3COOH v [NH4+

]= 0,1 M. Cch tnh gn ng trn l chp nhn c.V d 16:Tnh nng cn bng ca cc cu t v pH ca dung dch thu c khi trn 3 ml

HCOOH 0,03 M vi 6 ml CH3COOH 0,15 M. Bit CH3COOH c Ka=10-4,76; HCOOH c Ka=10

-

3,75.

Gii:Nng ban u ca cc cht:

101,09

3.03,0CMCHCOOH ; 21,09

6.15,03

CMC COOHCH

Cc cn bng xy ra:

HCOOH HCOO- + H+K1=10-3,75 (1)

CH3COOH CH3COO- + H+ K2= 10

-4,76 (2)

H2O H+ + OH- Kw= 10

-14 (3)

Ta c: K1.C1=10-5,75 K2.C2= 10

-5,76>> Kwnn trong dung dch cn bng (1) v (2) xy ra l

ch yu, c th b qua cn bng (3).

Ta c KP: [H+] = [HCOO-] + [CH3COO-]

376,575,522112211 10.875,11010....

CKCKH

H

CK

H

CKH

pH = 2,73


19/36



* Kim tra kt qu: 375,373,2

73,2

10.13,91010

10.01,0

HCOOH

099,01010

10.1,0

76,473,2

73,2

3

COOHCH

Tnh lp li: 376,4375,32 10.83,1099,0.1010.13,9.10 H pH= 2,74.

Vy kt qu tnh gn ng trn c th chp nhn c. T tnh tip nng cn bng ca cc

cu t trong dung dch.

4.2. Hn hp cc n baz yu.

V d 17: Tnh pH ca dung dch hn hp gm CH3COONa 0,0010 M(C1); HCOONa 0,020

M(C2) v NaCN 0,0050 M(C3). Bit CH3COOH c Ka=10-4,75; HCOOH c Ka=10

-3,75; HCN c

Ka=10-9,35.

Gii: Cc cn bng xy ra trong dung dch:

CH3COONa CH3COO- + Na+

0,0010 0,0010

HCOONa HCOO- + Na+0,020 0,020

NaCN Na+ + CN-

0,0050 0,0050

CH3COO- + H2O CH3COOH + OH

- Kb1=10-9,24(1) HCOO- +

H2O HCOOH + OH- Kb2=10

-10,25(2)

CN- + H2O HCN + OH- Kb3=10

-4,65(3)

H2O H+ + OH- Kw=10

-14 (4)

Ta c:Kb3.C3(=5.10-7,65) >> Kb1.C1(=10

-12,24) Kb2.C2(=2.10-12,25)> Kw nn mt cch gn ng ta

c th ch tnh ton theo cn bng(3).CN- + H2O HCN + OH

- Kb3=10-4,65

C0 0,0050

[ ] 0,0050-x x x

p dng LTDKL ta c: 010.510005,0

10 65,765,422

65,4

xxx

x

x=3,24.10-4= [OH-] pH= 10,51.

V d 18:Tnh pH ca dung dch thu c khi trn 20 ml KCN 0,25 M vi 30 ml NH30,1667 M.

Bit NH4+c Ka=10

-9,24; HCN c Ka= 10-9,35.

Gii:Nng ban u ca cc cht trong dung dch:

11,050

20.25,0CMCKCN ; 21,050

30.1667,03

CMCNH

Cc cn bng xy ra trong dung dch:

KCN K+ + CN-

0,1 0,1

CN- + H2O HCN + OH-Kb1=10

-4,65(1)

NH3 + H2O NH4+ + OH- Kb2=10

-4,76(2)


20/36



H2O H+ + OH- Kw=10

-14 (3)

V Kb1.C1(=10-5,65) Kb2.C2(=10

-5,76) >> Kw nn c th b qua cn bng(3), tnh ton theo cn

bng(1) v (2).

KP i vi cn bng (1) v (2): [OH-] = [HCN] +[NH4+]

376,565,521 10.21010..

21

OHOH

CK

OH

CKOH bb pH= 11,3

* Kim tra kt qu bng cch tnh lp:4.3.Hn hp cc axit yu v baz lin hp.

V d 19:Tnh pH ca h dung dch gm HCOOH 0,20 M v HCOONa 0,50 M. Bit HCOOH c

Ka=10-3,75.

Gii:Cch 1: Cc cn bng xy ra trong h dung dch:

HCOONa HCOO- + Na+

0,50 0,50

HCOOH HCOO- + H+ Ka= 10-3,75 (1)

HCOO-

+ H2O HCOOH + OH-

Kb= 10-10,25

(2)

V 775,475,3 1010.45,0

2,0.10.

b

aa C

CK nn dung dch c phn ng axit, tnh pH ca h theo

cn bng (1).

HCOOH HCOO- + H+ Ka= 10-3,75

C0 0,20 0,50

[ ] 0,20-x 0,50+x x

p dng LTDKL ta c: 010.2)105,0(2,0

)5,0(10 75,475,3275,3

xxx

xx

x= 7,11.10-5

= [H+

] pH = 4,15Cch 2: Tnh gn ng theo cng thc tnh pH ca dung dch m:

a

ba C

CpKpH lg =3,75 + lg(0,5/0,2) = 4,15

[H+]= 10-4,15


21/36



[ ] 9,8.10-4-x 1,00.10-3+x x

010.8,9)1010(10.8,9

)10(10 76,876,432

4

376,4

xx

x

xx

x= 1,65.10-5 pH = 9,22.

Cch 2: Tnh gn ng theo cng thc tnh pH ca dung dch m:

a

b

a C

CpKpH lg = 9,24 + lg(9,8.10-4/ 10-3)= 9,24-0,0088 =9,22

[H+] = 10-9,22Ka2>>Kw.nn b qua cn bng(2), (3), tnh ton da vo cn bng (1).

H2S H+ + HS- Ka1=10

-7

C0 0,1

[ ] 0,1-x x x

4101,0

10 42

7

pHHSHxx

x nn [OH-] = 10-10M.

Da vo cn bng (2) ta c:

92,122

292,12 10

.10

S

HS

SHM;


22/36



[H2S]=0,1-10-4 10-1M.

V d 23:Dung dch axit H3PO4 c pH= 1,5. Xc nh nng mol/l ban u ca dung dch

H3PO4. Bit H3PO4c Ka1=10-2,15; Ka2=10

-7,26;Ka3=10-12,32.


H3PO4 H+ + H2PO4

- Ka1= 10-2,23 (1)

H2PO4- H+ + HPO4

2-Ka2= 10-7,26 (2)

HPO42- H+ + PO43- Ka3= 10-12,32 (3)H2O H

+ + OH- Kw= 10-14 (4)

So snh: pH = 1,5 < pKa1= 2,23; trong dung dch cn bng (1) xy ra l ch yu:

H3PO4 H+ + H2PO4

- Ka1= 10-2,23

C0 x

[ ] x-10-1,5 10-1,5 10-1,5

2014,010

)10(10

5,1

25,123,2

x

x.

Vy nng ban u ca H3PO4l 0,2014 M.

Dng 2:Hn hp a axit v axit mnh.V d 24:Tnh pH v nng cn bng ca cc cu t trong h gm HCl 0,010 M v H2S 0,10

M. Bit H2S c Ka1= 10-7; Ka2=10

-12,9.

Gii: V yl mi trng axit nn s phn li ca nc l khng ng k..

Cc cn bng xy ra trong h:

HCl H+ + Cl-

0,010 0,010

H2S H+ + HS- Ka1= 10

-7 (1)

HS- H+ + S2- Ka2= 10-12,9 (2)

V Ka1>> Ka2 nn cn bng (1) l ch yu.H2S H

+ + HS- Ka1= 10-7

C0 0,10 0,010

[] 0,10-x 0,010+x x

.101,0

)01,0( 7

x

xxGi s x


23/36



Do Ka1>>Ka2>>Ka3v Ka Ka1v 2 axit cng nng nn phi tnh theo c cn bng (1) v (2).

Nu chn mc khng l H3PO4v HSO4-th KP l:

h

HSOK

h

POHKSOPOHHh a

a

4432

442

..1

443.1 HSOKPOHKh aa (5)

- Bc 1: v C= 0,010 Ka Ka1

Chn [H3PO4]0= [HSO4-]0= C/2=0,005 M v thay vo (5) tnh h1:

399,115,21 10.3,90050,0.100050,0.10

h

Thay gi tr h1vo biu thc tnh [H3PO4] v [HSO4-] :

[H3PO4]1= MhK

hC

a

315,23

3

10.68,51010.3,9

10.3,9.01,0.

1

[HSO4-]1= M

hK

hC

a

3

99,13

3

10.67,41010.3,9

10.3,9.01,0.

- Bc 2: Thay gi tr [H3PO4]1v [HSO4-]1vo (5) tnh h2:

13399,1315,2

2 10.4,910.76,4.1010.68,5.10 hh

Vy kt qu tnh lp gn ng. Vy pH = -lgh2= 2,03.

5.2. Cn bng trong dung dch a baz

Dng 1:Dung dch cha mt a baz

V d 26:Tnh pH ca dung dch Na2S 0,010 M. Bit H2S c Ka1= 10-7; Ka2=10

-12,92.


Na2S 2Na+ + S2-

S2- + H2O HS-+ OH- Kb1= 10

-14/10-12,92= 10-1,08(1)

HS- + H2O H2S + OH- Kb2= 10

-14/10-7= 10-7 (2)

H2O H+ + OH- Kw= 10

-14 (3)

V Kb1>>Kb2>>Kwnn trong dung dch cn bng (1) xy ra l ch yu.

S2- + H2O HS-+ OH- Kb1= 10

-1,08

C0 10-2

[] 10-2-x x x

p dng LTDKL ta c: 0101010

10 08,308,122

208,1

xx

x

x

x= 9.10-3= [HS-] = [OH-]

[H+] = 1,1.10-12 pH = 11,95.

V d 27:Tnh khi lng mui Na2S phi cho vo 1 lt nc c dung dch c pH = 11,5.

Bit H2S c Ka1= 10-7; Ka2=10

-12,92.

Gii : Cc cn bng xy ra trongdung dch l:

Na2S 2Na+ + S2-

a a

S2- + H2O HS-+ OH- Kb1= 10

-14/10-12,92= 10-1,08(1)

HS- + H2O H2S + OH- Kb2= 10

-14/10-7= 10-7 (2)

H2O H+ + OH- Kw= 10

-14 (3)



24/36



S2- + H2O HS- + OH- Kb1= 10

-1,08

C0 a

[ ] a-x x x

xa

x

208,110 trong x= [OH-] =10-14/ 10-11,5= 10-2,5

a = 3,28.10-3= SNaC 2 gm SNa 256,078.10.28,33

2

Dng 2:Hn hp a baz v baz mnhV d 28:Trn 10,00 ml dung dch NaOH 8,00.10-3M vi 30,00 ml dung dch H2S 1,00.10

-3M.

Tnh pH ca dung dch thu c.

Bit H2S c Ka1= 10-7; Ka2=10

-12,92.

Gii:

y l bi ton pha trn gia a axit (H2S) vi baz mnh nn c phn ng xy ra, chng ta cn

xc nh thnh phn gii hn, t m t cc cn bng xy ra trong dung dch v tnh pH.

- Nng ban u ca cc cht trong dung dch: MCNaOH3

3

10.240

10.10.8

MC SH3

310.75,0

4030.10.1

2

NaOH + H2S NaHS + H2O

0,75.10-3 0,75.10-3 0,75.10-3

NaOH + NaHS Na2S + H2O

0,75.10-3 0,75.10-3 0,75.10-3

TPGH gm: Na2S 0,75.10-3M; NaOH d 0,5.10-3 M.

Vy dung dch l hn hp a baz ( S2-) v baz mnh, nn ta c cc cn bng xy ra:

S2- + H2O HS- + OH- Kb1= 10

-1,08 (1)

HS-

+ H2O H2S + OH-

Kb2= 10-7

(2)H2O H

+ + OH- Kw= 10-14 (3)


S2- + H2O HS- + OH- Kb1= 10

-1,08

C0 0,75.10-3 0,5.10-3

[ ] 0,75.10-3-x x 0,5.10-3+x

p dng LTDKL ta c:x

xx

3

308,1

10.75,0

)10.5,0(10

Gii PT bc 2 ta c: x = 7,4.10-4 [OH-]=1,24.10-3 [H+]=8,06.10-12

pH= 11,09.VI. Cn bng trong dung dch cc cht lng tnh

6.1. Mui axit c to bi baz mnh v axit yu

V d 29:Tnh pH ca dung dch NaHCO31 M.

Bit H2CO3c Ka1=10-6,35; Ka2=10

-10,33.


NaHCO3 Na+ + HCO3

-

HCO3- H+ + CO3

2- Ka2= 10-10,33 (1)

HCO3- + H2O H2CO3 + OH

- Kb2= 10-7,65 (2)


25/36



H2O H+ + OH- Kw= 10

-14 (3)

V Kb2>Ka2 nn dung dch c phn ng baz.

Kim tra cc iu kin gn ng ta c:

Kw


26/36



T hp(2) v (3) ta c:

NH4+ + Ax- NH3 + HAx K= 10

-4,48

C0 0,1 0,1

[ ] 0,1-x 0,1-x x x

p dng LTDKL ta c:2

248,4

)1,0(10

x

x

vi x >

Kb=10-9,24>>Kw nn cn bng trong dung dch ch yu l cn bng phn li ca dng axit.Cch 1: Xt cn bng xy ra trong dung dch:


-4,76

C0 0,05 0,05

[ ] 0,05-x 0,05+x x

p dng LTDKL ta c : x=10-4,76=[H+] pH= 4,76.

Cch 2: p dng cng thc tnh gn ng:

.76,405,0

05,0lg76,4lg

a

ba C

CpKpH


27/36



Ta c [H+] = 10-4,76Ka>>Kw

nn cn bng ca dng baz chim u th.

Cch 1: NH3 + H2O NH4+

+ OH-

Kb= 10-4,76

C0 0,1 0,005

[ ] 0,1-x 0,005+x x

51,1010.08,310.25,3101,0

)005,0( 11476,4

pHHOHx

x

xxKb

Cch 2: Tnh gn ng:

54,10005,0

1,0lg24,9lg

a

ba C

CpKpH

Ta c [H+]=10-10,54 Kb=10-9,24>>

Kwnn cn bng dng axit chim u th.

p dng cng thc tnh gn ng ta c:

76,41,0

1,0lg76,4lg

a

ba C

CpKpH

Vy ta c [H+]=10-4,76


28/36



CH3COOH + NaOH CH3COONa + H2O

0,10 0,001

0,10-0,001 - 0,001

TPGH: CH3COOH 0,099 M ; CH3COONa 0,101 M

Cn bng xy ra: CH3COONa CH3COO-

+ Na+

0,101


-4,76

C0 0,099 0,101

[ ] 0,099-x 0,101+x x

76,410099,0

)101,0( x

xxvi x


29/36



x x x

H2PO4- + H+ H3PO4

x.17,5

17,4 x.

17,5

17,4 x.

17,5

17,4

Tng nng H+phn ng l: 2,807x [H+] d = 0,08-2,807x x= 0,01723

Vy trong 50ml dung dch ban u th )(0517,03)( 43 MxC PONaM

2. Ta c pH = 7,26 = pK2ca H3PO4, vy H3PO4v HCl b trung ho ht, H2PO4-b trung ho1/2.

Cc phn ng xy ra:

H+ + OH- H2O

10-1,5 10-1,5

H3PO4 + OH- H2PO4

-

C0 0,0139 3,33.10-3

C - 0,0172

H2PO4- + OH- HPO4

2-

C0

0,0172C 8,6.10-3 8,6.10-3

1,0

100).10.6,80139,010( 35,1 ddNaOHV = 54,123(ml)

3. pH = 4, xt cc cn bng sau y:

H3PO4 H+ + H2PO4

-

017,0

10

1023,2

4

142

43

K

H

POH

POH

H2PO4- H+ + HPO4

-

7,181910

1026,7

4

22

4

42

K

H

HPO

POH

H2O + CO2 H+ + HCO3

- K1= 10-6,35

87,22310

1035,6

4

33

32

K

H

HCO

COH

Nh vy thnh phn ch yu ca dung dch thu c l H2PO4-v H2O, CO2.

Cc phn ng xy ra: CO32-+ H+ HCO3

-

CO32-+ H3PO4 HCO3

- + H2PO4-

HCO3-

+ H+

H2O + CO2HCO3

- + H3PO4 H2O + CO2+ H2PO4-

V d 39: ( thi HSGQG nm 2002-2003)

Dung dch bo ha H2S c nng 0,100 M. Hng s axit ca H2S:K1= 1,0.10-7; K2= 1,3.10

-13.

a) Tnh nng ion sunfua trong dung dch H2S 0,100 M khi iu chnh pH = 2,0.

b) Mt dung dch Acha cc cation Mn2+, Co2+, v Ag+vi nng ban u ca mi ion ubng 0,010 M. Ho tan H2S vo An bo ho v iu chnh pH = 2,0 th ion no to kt ta.

Cho: TMnS= 2,5.10-10; TCoS= 4,0.10

-21; 5010.3,62

SAgT .

Gii:


30/36



a) Tnh nng ion S2trong dung dch H2S 0,100 M; pH = 2,0.

C = H2S= 0,1 M ; H2S= 101; H+ = 102

H2S (k) H2S (aq)

H2S (aq) H+ + HS K1= 1,0.10

-7

HS H+ + S2 K2= 1,3.10-13

H2S (aq) 2H+ + S2 K =K1.K2=1,3 10

20

20

2

22

10.3,1

SH

SHK ; S2=1,3.1020.

22

H

SH=1,3.1020.

22

1

)10(

10

=1,3.1017(M)

b) Ta c: Mn2+S2= 102.1,3.1017= 1,3.1019TMnS= 2,5.1010,khng c kt ta.

Co2+S2 = 102.1,3.1017= 1,3 1019 TCoS= 4,0.1021 to kt ta CoS

Ag+2S2= (102)2.1,3.1017 = 1,3.1021 5010.3,62

SAgT to kt ta Ag2S

V d 40: ( thi chn T d thi Olympic quuc t nm 2003)1.Hirazin(N2H4) l mt baz hai nc ( ibaz).

a)Tnh cc hng s baz ca N2H4.

b)Trn 10,00 ml H2SO40,400 M vi 40,00 ml N2H40,100 M ri thm vi git phenolphtalein.Tnh th tch NaOH 0,200 M cn chun hn hp n va xut hin mu ta (pH ~ 10);

Nu chun hn hp n pH ~ 8 th th tch NaOH 0,200 M cn l bao nhiu?

Cho bit iaxit N2H62+c pKa1= 0,27; pKa2= 7,94. HSO4

-c pKa= 2,00.

2. a) Hirazin c tnh kh mnh. Cho bit 250C, th in cc tiu chun E0ca cp N2/ N2H5+

bng -0,23 V. Tnh E0ca cc cp N2/ N2H62+v cp N2/ N2H4.

b)Hy thit lp phng trnh ph thuc E-pH ca cc cp oxi ho- kh ca hirazin pH < 0;

pH = 2,0- 7,0 v pH > 9,0.

c)Vit phng trnh ion ca phn ng xy ra gia hirazin vi KMnO4 pH = 0 v pH = 9,0.

Bit rng cc pH ny, hirazin u b oxi ho thnh N2.Gii:1.a) Tnh cc hng s baz ca N2H4.

N2H4+ H2O N2H5++ OH- Kb1

06,694,7

14

)(

1010

10

622

1

HNa

wb K

KK

N2H5++ H2O N2H6

++ OH-Kb2

73,1327,0

14

)(

1010

10

621

2

HNa

wb K

KK

b) * Trn dung dch N2H4v dung dch H2SO4c dung dch mi c nng u ca cc cht

l: H2SO40,08M; N2H40,08M.

- Phn ng xy ra: N2H4+ H2SO4 N2H5++ HSO4

-

0,08 0,08 0,08 0,08

- Dung dch thu c c cha N2H5+0,08M v HSO4

-0,08M

* Chun hn hp n pH = 10 ngha l dung dch phi cha N2H4.

HSO4-+ OH- H2O + SO4

2-

N2H5++ OH- N2H4+ H2O

V(ddNaOH)= (0,08+0,08).50/ 0,2 = 40(ml)

H2S


31/36



* Chun hn hp n pH = 8 pKa2ca N2H6+, nh vy trung ho na lng N2H5

+:

V(ddNaOH)= (0,08+0,04).50/ 0,2= 30( ml)

2.a) Tnh E0

* CpN2/N2H62+:

N2+ 5H++ 4e N2H5

+ E0= -0,23V

N2H5++ H+ N2H6

2+K =100,27

N2+ 6H++ 4e N2H6

2+ VEHNN

226,00/ 622

* Cp N2/N2H4:

N2+ 5H++ 4e N2H5

+ E0= -0,23V

N2H5+ N2H4+ H

+ K = 10-7,94

N2+ 4H++ 4e N2H4 VE HNN 348,0

0

/ 422

b) * pH < 0, ton b hirazin dng N2H62+, ta c cp N2/N2H6

2+

N2+ 6H++ 4e N2H6

2+

vi 12 Np ; [N2H62+] = 1, ta c: pHHE 0888,0226,0lg40592,0226,06

* pH = 2,07,0 th dng ch yu ca hirazin l N2H5+ta c cp N2/ N2H5

+

N2+ 5H++ 4e N2H5

+ E0= -0,23V

pHHE 074,023,0lg4

0592,023,0

5

* pH > 9 dng tn ti ch yu l N2H4ta c cp N2/N2H4

N2+ 4H++ 4e N2H4 E

0= -0,348V

pHE 0592,0348,0

c) Phn ng ca hirazin vi KMnO4- pH = 0 : 5N2H62++ 4MnO4

-+ 2H+ 5N2+ 4Mn2++ 4H2O

- pH = 9: N2H4+ 4MnO4-+ 4OH- N2+ 4MnO4

2-+ 4H2O

V d 41: ( thi HSGQG nm 2007)

Mt dung dch c 3 cht HCl, BaCl2, FeCl3cng nng 0,0150M. Sc kh CO2vo dung dch

ny cho n bo ho. Sau thm t t NaOH vo dung dch n nng 0,120M. Cho bit:

nng CO2trong dung dch bo ho l 3.10-2M; th tch dung dch khng thay i khi cho CO2

v NaOH vo. cc hng s: pKa ca H2CO3 l 6,35 v 10,33; pKSca Fe(OH)3 l 37,5 v ca

BaCO3l 8,30; pKaca Fe3+l 2,17.

Tnh pH ca dung dch thu c.

Gii:Khi cho kh CO2vo hn hp gm H

+0,0150M; Ba2+0,0150M; Fe3+0,0150M c cc qu trnh

sau:

CO2+ H2O HCO3-+ H+ Ka1=10

-6,35

HCO3- CO3

2- + H+ Ka2= 10-10,33

HCl H+ + Cl-

Fe3+ + H2O FeOH2+ + H+ Ka= 10

-2,17

Dung dch c mi trng axit mnh( v c HCl v Fe3+), s in li ca CO2l khng ng k( v

nng CO32-v cng b) nn khng c kt ta BaCO3to thnh.


32/36



Khi thm NaOH xy ra cc phn ng:

- Trung ho HCl: H+ + OH- H2O

0,015 0,120

- 0,105

- Kt ta Fe(OH)3: Fe3+ + 3OH- Fe(OH)3

0,015 0,105

- 0,06 0,015- Phn ng vi CO2: CO2 + 2OH

- CO32- + H2O

3.10-2 0,06

- - 0,030

- Kt ta BaCO3: Ba2+ + CO3

2- BaCO3

0,015 0,030

- 0,015 0,015

Thnh phn hn hp kt ta c: Fe(OH)3 0,0150 mol; BaCO30,0150 mol.

Trong dung dch c: CO32-0,015M; Cl-; Na+; H2O.

Cc cn bngxy ra:

H2O H+ + OH- Kw= 10

-14 (1)

Fe(OH)3 Fe3+ + 3OH- KS1= 10

-37,5 (2)

BaCO3 Ba2+ + CO3

2- KS2= 10-8,30 (3)

CO32- + H2O HCO3

-+ OH-Kb1= 10-14/10-10,33= 10-3,67 (4)

So snh cho thy cn bng (4) l cn bng quyt nh pH ca dung dch( v OH-do nc in li

v do Fe(OH)3tan ra l rt b), nng CO32-do BaCO3tan ra khng ng k( v c d CO3

2-t

dung dch).

Tnh pH theo (4):

CO32- + H2O HCO3

- + OH- Kb1= 10-3,67

C0 0,015

[ ] (0,015-x) x x

23,1110.69,110015,0

367,32

pHMOHxx

x

* Kim tra kt qu:

[OH-] =

H323,11

14

1010

10( do nc in li khng ng k)

[Fe3+] = 29338

5,37

10)10(

10

rt nh, do OH-do Fe(OH)3tan ra khng ng k. [Ba2+] =

.015,010.2,4)10.7,1015,0(

10 73

3,8

V vy [CO32-] do BaCO3tan ra khng ng k. Vy

cch gii trn l chnh xc.

C. BI TP T LUYN

Cu 1: Tnh pH ca dung dch thu c trong cc hn hp sau:-10ml dung dch axit axtic (CH3COOH) 0,10M trn vi 10ml dung dch HCl c pH = 4,0 - 25ml dung dch axit axtic c pH = 3,0 trn vi 15ml dung dch KOH c pH = 11,0- 10ml dung dch axit axtic c pH = 3,0 trn vi 10ml dung dch axit fomic (HCOOH) c pH =3,0.


33/36



Bit pKa ca CH3COOH v HCOOH ln lt l 4,76 v 3,75Cu 2 :1/ Mt axit yu n chc ho tan vo nc, nng C (mol/l), hng s axit K, nng [H +] lccn bng a(mol/l)

a)Chng minh :2

aC a

K

b)T gii thch ti sao dung dch ca mt n axit yu cng long th pH ca dung dch cng

tng.2/Trong mt dung dch 2 axit yu HA1v HA2c hng s cn bng khc nhau.a)Tnh nng [H+] trong dung dch 2 axit theo hng s cn bng v nng ca 2 axit.b)p dng : Trong 1 dung dch 2 axit CH3COOH 2.10

-3(mol/l) v C2H5COOH 1,92.10-2 (mol/l).

Tnh pH ca dung dch 2 axit .Cu 3. Tnh th tch dung dch NaOH 0,01 M cn dng trung ha hon ton 10ml dung d chH2SO4c pH = 2. Bit HSO4

-c pKa= 2.Cu 4:1. Tnh pH ca dung dch H2C2O40,01M.2. Cho t t dung dch C2O4

2-vo dung dch cha ion Mg2+0,01M v Ca2+0,01M.2.1. Kt ta no xut hin trc.2.2.Nng ion th nht cn li bao nhiu khi ion th hai bt u kt ta.

3. Tnh pH ca dung dch 0,001moL CaC2O4tan ht trong 1 Lt dung dch .Bit H2C2O4c cc hng s axt tng ng l pK1= 1,25; pK2= 4,27Tch s tan ca CaC2O4l 10

8,60; MgC2O4l 10- 4,82

Cu 5:Tnh pH v nng mol ca Cr 24O , Cr2 2

7O trong dung dch K2Cr2O7 0,01M v

CH3COOH 0,1M. Cho: OHCOCH 3K = 1,8.10-5

HCr 4O + H2O Cr 2

4O + H3O+ pK2= 6,5

2HCr 4O Cr2 2

7O + H2O pK1= -1,36

Cu 6: Dung dch A gm c H2SO40,05 M; HCl 0,18 M; CH3COOH 0,02 M. Thm NaOH vodung dchA n nng caNaOH thm vo l 0,23 M th dng thu c dung dchA1.

a) Tnh nng cc cht trong dung dchA1.

b)

Tnh pH ca dung dchA1.c) Tnh in ly ca CH3COOH trong dung dchA1.

Cho: Ka(HSO 4 )= 10-2 ; Ka(CH3COOH) = 10

-4,75

Cu 7:Trn 100 ml dd HCOOH 0,1M vi 100 ml dd NaOH 0,05M c 200 ml dd A. Tm pH ca dd A.- pH ca dd A s thay i nh th no khi thm vo dd 0,001 mol HCl hoc 0,001 mol

NaOH.- T cc kt qu trn hy nhn xt.

Cu 8: Ha tan 0,1mol NH4Cl vo 500ml nc.a. Vit phng trnh phn ng v biu thc tnh Ka

b. Tnh pH dung dch trn bit KaNH+4= 5.10

10

Cu 9:Tr s pH ca nc nguyn cht l 7,0; trong khi nc ma t nhin c tnh axit yu do

s ha tan ca CO2trong kh quyn. Tuy nhin trong nhiu khu vc nc ma c tnh axit mnhhn. iu ny do mt s nguyn nhn trong c nguyn nhn t nhin v nhng nguyn nhnxut pht t hot ng ca con ngi. Trong kh quyn SO2v NO b oxi ha theo th t thnhSO3v NO2, chng phn ng vi nc chuyn ha thnh axt sunfuric v axt nitric. Hu qul to thnh ma axt vi pH trung bnh khong 4,5.Lu hunh dioxit l mt oxit hai chc trong dung dch nc. Ti 250C :

SO2 (aq) + H2O (l) HSO3-(aq) + H

+( aq) Ka1= 10

-1,92MHSO3

-(aq) SO3

-(aq) + H

+( aq) Ka2= 10

-7,18MTt c cc cu hi sau u xt 250C1. tan ca SO2l 33,9 L trong 1 L H2O ti p sut ring phn ca SO2bng 1 bar.


34/36



1.1.Tnh nng ton phn ca SO2trong nc bo ha kh SO2(b qua s thay i th tchdo s ha tan SO2).

1.2.Tnh thnh phn phn trm ca ion HSO3-.

1.3.Tnh pH ca dung dch.2.Nh tng git Br2n d vo dung dch SO20,0100 M, ton b SO2b oxi ha thnh SO4

2-. Br2d c tch ra bng cch sc vi kh N2 .

Vit mt phng trnh phn ng ca qu trnh. Tnh nng H+ trong dung dch thu c.Bit pKa(HSO4

-) = 1,99.

Cu 10: Cho 0,01 mol NH3, 0,1 mol CH3NH2v 0,11 mol HCl vo nc c 1 lt dung dch.Tnh pH ca dung dch thu c?

Cho: pK4

NH = 9,24; pK

3 3CH NH

= 10,6; pK2H O

= 14.

Cu 11: Dung dch K2CO3 c pH=11 (dung dch A). Thm 10ml HCl 0,012M vao 10ml ddA tathu c ddB. Tnh pH cua ddB. Biet rng H2CO3 c pk1=6,35 va pk2=10,33.CU 12:1)Nu khi nim dung dch m? Cho 2 v d.2)Dung dch X l dung dch hn hp gm axit yu HA 0,1M v NaA 0,1M.

a) tnh pH ca dung dch Xb) thm vo 1 lt dung dch X trn

b-1: 0,01 mol HCl

b-2: 0,01 mol NaOHHy tnh pH ca dung dch thu c trong mi trng hp bit Ka HA= 6,8 10

-4Cu 13: Tnh pH ca dung dch CH3COONH40,4 M bit rng KA (CH3COOH) = 1,8.10

-5; KB(NH3)= 1,6 .10

5

Cu 14: c dung dch m c pH = 8,5, ngi ta trn dung dch HCl 0,2M vi 100ml dungdch KCN 0,01M. Tnh th tch dung dch HCl 0,2M c dng, bit rng HCN c KA =4,1.10-10.Cu 15: Trn 2 th tch bng nhau ca 2 dung dch CH3COOH 0,2M v NaOCl 0,2M . Tnhnng ca cc cu t trong dung dch khi phn ng trng thi cn bng . Ka(CH3COOH) =1,75.10-5v Ka(HOCl) = 5.10

-8

Cu 16:Ha tan ht 1,25 gam axit HA vo nc thnh 50ml dung dch. chun ht lngaxit ny cn dng 41,20ml dung dch NaOH 0,09M. Hy xc nh pH ca dung dch thu c tiim tng ng ca php chun ? Bit rng nu thm 8,24ml dung dch NaOH trn volng axit ban u th pH ca dung dch thu c lc l 4,3Cu 17:1. C mt dung dch axit HA v HX, bit nng ca axit HX trong dung dch l 2 .10 3 M.Tnh nng ca axit HA trong dung dch sao cho in ly ca HX l bng 0,08.

Cho KHA= 1,3 . 10 5 v KHX= 1,8 .10

5

2. 2.1. Tnh nng ion S2 v pH ca dung dch H2S2 0,010M.

2.2.Khi thm 0,001 mol HCl vo 1 lit dung d ch H2S 0,010M th nng ion S2 bng bao

nhiu?Cho hng s axit ca H2S :

7A 10K 1

v 12,92A 10K 2 .

Cu 18: Tnh pH ca dung dch NH4HCO30,1M. Bit:

NH3 c 76,4b 10K ; H2CO3c 35,61a 10K v 33,102a 10K

**************************************************


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PHN III: KT LUN

Vic hc tp sng kin kinh nghim s thu c kt qu tt nu m bo cc yu cu sau:

Hc sinh phi c trnh nhn thc v t duy tng i tt. Xut pht t i tng hc u

l hc sinh kh gii, nn kh nng tip thu kin thc kh nhanh v chc chn. l tin rt tt

c th truyn th mt khi lng kin thc trong cng mt n v thi gian nhiu hn so vi

hc sinh khc. Gio vin cn bit tn dng c hiu qu nhng kh nng , chng hn, bng cch

a ti liu, yu cu hc sinh t nghin cu trc sau trnh by, a ra nhn xt, kt qu thu

c trong tit hc chuyn Nh vy s gip hc sinh lnh hi kin thc su sc hn, to iu

kin cc em bc u tp dt nghin cu khoa hc.

1. Kt qu thc tinQua thc t,ti c tham gia mt s bui dy i tuyn ; trc tip ging dy sng kin kinh

nghim ny trong cc tit chuyn bi dng hc sinh d thi hc sinh gii quc gia mn Ha 12

ti trng THPT chuyn Hng Yn , vi lng kin thc va phi v h thng v d ph hp

gip hc sinh tip thu kh tt, kch thch v pht huy kh nng t duy, vn dng tng hp kin thc

mt cch lgic, say m t gic hc tp, gi m c tm ti sng to khoa hc.

Hc sinh i tuyn lp 12 d thi hc sinh gii quc gia t tin hn khi gp cc bi tp v

phn ng axitbaz.

2. Bi hc kinh nghimSng kin kinh nghim ny c ging dy cho cc th h hc sinh cc lp chuyn, nn cn

c thng xuyn trao i, cp nht lin tc cc bi ton lin quan nphn ng axitbaz.

3. Kin ngh, xutTrn y l hthng cu hi v bi tp phn Phn ng axit baz m ti p dng

trong ging dy. ti t-ng i ph hp vi yu cu v mc ch ging dy, bi d-ng hc sinhkh, gii tr-ng chuyn chun b d thi hc sinh gii cc cp . ti ny cng c thd ng lmti liu hc tp cho hc sinh c c l p chuyn Ho hc v ti liu tham kho cho c c thy c gi otrong ging dy v bi d-ng hc sinh gii Ho hc bc THPT gp phn nng cao cht l-ng

ging dy v hc tp mn Ho hc.


36/36


Tuy nhin, y ch l mt phn rt nh trong ch-ng trnh n luyn cho hc sinh chun btham gia vo c c k thi hc sinh gii c c cp. Vvy, ti rt mong c c Thy , C ng nghip gp kin cho ti vchuyn ny v c ng nhau ph t trin sang c c chuyn kh c hc trchuyn Ho ngy cng c nhiu ti liu hc tp mt c ch hthng hn.

Ti liu tham kho

[1]. Mt s vn chn lc tp 2 Nguyn Tinh Dung

[2]. Ho hc phn tch - Nguyn Tinh Dung

[3]. Cs lthuyt ho hc-Nguyn Hnh

[4]. Cs lthuyt c c qu trnh ho hc- V ng

[5]. Cs lthuyt ho hc- L Mu Quyn

[6]. Cs lthuyt c c phn ng ha hc Trn Th , ng Trn Phch

30 nguyenthinhung thpt chuyenhungyen hoa 5191

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