1 chemical equilibrium & acids & bases chemistry i

11

Chemical Equilibrium & Acids & bases

Chemistry I

22

Reversible Reactions

• Some reactions do not go to completion as we have assumed–They may be reversible – a reaction in which the conversion of reactants to products and the conversion of products to reactants occur simultaneously

• Forward: 2SO2(g) + O2(g) → 2SO3(g)

• Reverse: 2SO2(g) + O2(g) ← 2SO3(g)

33

Reversible Reactions

• The two equations can be combined into one, by using a double arrow, which tells us that it is a reversible reaction:

2SO2(g) + O2(g) ↔ 2SO3(g)

A chemical equilibrium occurs, and no net change occurs in the actual amounts of the components of the system.

44Reversible Reactions

• Even though the rates of the forward and reverse are equal, the concentrations of components on both sides may not be equal–An equlibrium position may be shown:

A B or A B 1% 99% 99% 1%Note the emphasis of the arrows direction It depends on which side is favored; almost all reactions

are reversible to some extent

55

Le Chatelier’s Principle

• The French chemist Henri Le Chatelier (1850-1936) studied how the equilibrium position shifts as a result of changing conditions

• Le Chatelier’s principle: If stress is applied to a system in equilibrium, the system changes in a way that relieves the stress

66


• What items did he consider to be stress on the equilibrium?

1) Concentration

2) Temperature

3) Pressure

• Concentration – adding more reactant produces more product, and removing the product as it forms will produce more product

Each of these will now be discussed in detail

77


• Temperature – increasing the temperature causes the equilibrium position to shift in the direction that absorbs heat• If heat is one of the products (just like a

chemical), it is part of the equilibrium• so cooling an exothermic reaction will

produce more product, and heating it would shift the reaction to the reactant side of the equilibrium: C + O2(g) → CO2(g) + 393.5 kJ

88


• Pressure – changes in pressure will only effect gaseous equilibria• Increasing the pressure will usually

favor the direction that has fewer molecules

N2(g) + 3H2(g) ↔ 2NH3(g)

• For every two molecules of ammonia made, four molecules of reactant are used up – this equilibrium shifts to the right with an increase in pressure

99

Predicting shifts in equilibrium

• H2(g) + I2(s) ↔ 2HI(g)(colorless) (purple) (colorless)

∆H = + 52 kJ/mol

1. In which direction will the equilibrium shift if more H2 is added?

2.In which direction will the equilibrium shift if HI is removed?

3.In which direction will the equilibrium shift if heat is added to the reaction system?

1010

Predicting equilibrium shifts

• C(s) + H2O(g) ↔ CO(g) + H2(g)

• Which way will the equilibrium shift if pressure is increased?

1111

Equilibrium Constants: Keq• Chemists generally express the position of equilibrium in terms of numerical values, not just percent

–These values relate to the amounts (Molarity) of reactants and products at equilibrium

–This is called the equilibrium constant, and abbreviated Keq

1212Equilibrium Constants

• consider this reaction (the capital letters are the chemical, and the lower case letters are the balancing coefficient):

aA + bB cC + dD

–The equilibrium constant (Keq) is the ratio of product concentration to the reactant concentration at equilibrium, with each concentration raised to a power (which is the balancing coefficient).

1313

Equilibrium Constants• consider this reaction:

aA + bB cC + dD

–Thus, the “equilibrium constant expression” has this general form:

[C]c x [D]d

[A]a x [B]b

(brackets: [ ] = molarity concentration)

KKeqeq ==

Note that Keq has no units on the answer; it is only a number because it is a ratio

1414

Equilibrium Constants

• the equilibrium constants provide valuable information, such as whether products or reactants are favored:

if Keq > 1, products favored at equilibrium

if Keq < 1, reactants favored at equilibrium

1515

Expressing and Calculating Keq

• A colorless gas dinitrogen tetroxide (N2O4) and the dark brown gas nitrogen dioxide (NO2) exist in equilibrium with each other.

N2O4 ↔ 2 NO2

1.00 Liter of a gas mixture at equilibrium at 10oC contains 0.0045 mol of N2O4 and 0.030 mol of NO2. Write the expression for the equilibrium constant and calculate the equilibrium constant (Keq) for the reaction.

1616

Answer

• Keq = [NO2]2

[N2O4]

Keq = (0.030)2 = 0.20 (0.0045)

Does this reaction favor the reactants or the products?

1717The Chemistry The Chemistry of Acids and of Acids and BasesBases

The Chemistry The Chemistry of Acids and of Acids and BasesBases• `

1818

Acid and BasesAcid and Bases

1919


2020


2121Acids

Have a sour taste. Vinegar is a solution of acetic acid. CitrusHave a sour taste. Vinegar is a solution of acetic acid. Citrusfruits contain citric acid.fruits contain citric acid.

React with certain metals to produce hydrogen gasReact with certain metals to produce hydrogen gas..

React with carbonates and bicarbonates to produce carbon React with carbonates and bicarbonates to produce carbon dioxide gasdioxide gas

Have a bitter taste.Have a bitter taste.

Feel slippery. Many soaps contain bases.Feel slippery. Many soaps contain bases.

Bases

2222

Some Properties of Acids

Produce H+ (as H3O+) ions in water (the hydronium ion is a

hydrogen ion attached to a water molecule)

Taste sour

Corrode metals

Electrolytes

React with bases to form a salt and water

pH is less than 7

Turns blue litmus paper to red “Blue to Red A-CID”

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Anion Ending Acid Name

-ide hydro-(stem)-ic acid

-ate (stem)-ic acid

-ite (stem)-ous acid

Acid Nomenclature Review

No OxygenNo Oxygen

w/Oxygen w/Oxygen

An easy way to remember which goes with which…An easy way to remember which goes with which…

““In the cafeteria, you In the cafeteria, you ATEATE something something ICICky”ky”

2424Acid Nomenclature Flowchart

h yd ro - p re fix-ic en d in g

2 e lem en ts

-a te en d in gb ecom es-ic en d in g

-ite en d in gb ecom es

-o u s en d in g

n o h yd ro - p re fix

3 e lem en ts

AC ID Ss ta rt w ith 'H '

2525

• HBr HBr (aq)(aq)

• HH22COCO33

• HH22SOSO33

hydrohydrobromicbromic acidacid

carboncarbonicic acidacid

sulfursulfurousous acidacid

Acid Nomenclature Review

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Name ‘Em!

• HI HI (aq)(aq)

• HCl HCl (aq)(aq)

• HH22SOSO33

• HNOHNO33

• HCIOHCIO44

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Some Properties of Bases

Produce OHProduce OH-- ions in water ions in water

Taste bitter, chalkyTaste bitter, chalky

Are electrolytesAre electrolytes

Feel soapy, slipperyFeel soapy, slippery

React with acids to form salts and waterReact with acids to form salts and water

pH greater than 7pH greater than 7

Turns red litmus paper to blue “Turns red litmus paper to blue “BBasic asic BBlue”lue”

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Some Common Bases

NaOHNaOH sodium hydroxidesodium hydroxide lyelye

KOHKOH potassium hydroxidepotassium hydroxide liquid soapliquid soap

Ba(OH)Ba(OH)22 barium hydroxidebarium hydroxide stabilizer for plasticsstabilizer for plastics

Mg(OH)Mg(OH)22 magnesium hydroxidemagnesium hydroxide “MOM” Milk of “MOM” Milk of

magnesiamagnesia

Al(OH)Al(OH)33 aluminum hydroxidealuminum hydroxide Maalox (antacid)Maalox (antacid)

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Dissociation

• Strong acids and bases completely dissociate in water. Acids dissociate to form H+ ions. Bases dissociate to form OH-1 ions. The more ions in solution, the stronger the acid or base it will be. Weak acids and bases partially dissociate.

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Dissociation Reactions

• Write dissociation reactions for the following acids and bases in water.

– HCl (l) → H+(aq) + Cl-1 (aq)

– NaOH (s) → Na+ (aq) + OH- (aq)

– Ca(OH)2 (s)→ Ca+2 (aq) + 2OH-1 (aq)

– H2SO4 (l) → 2H+(aq) + SO4-2(aq)

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Acid/Base definitions

• Definition #1: Arrhenius (traditional)

Acids – produce H+ ions (or hydronium ions H3O+)

Bases – produce OH- ions

(problem: some bases don’t have hydroxide ions!)

3232Arrhenius acid is a substance that produces H+ (H3O+) in water

Arrhenius base is a substance that produces OH- in water

3333

Acid/Base Definitions

• Definition #2: Brønsted – Lowry

Acids – proton donor

Bases – proton acceptor

A “proton” is really just a hydrogen atom that has lost it’s electron!

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A Brønsted-Lowry acid is a proton donorA Brønsted-Lowry base is a proton acceptor

acidconjugate

basebase conjugate

acid

3535

ACID-BASE THEORIESACID-BASE THEORIESACID-BASE THEORIESACID-BASE THEORIES

The Brønsted definition means NHThe Brønsted definition means NH33 is is aa BASEBASE in water — and water is in water — and water is itself anitself an ACIDACID

BaseAcidAcidBaseNH4

+ + OH-NH3 + H2OBaseAcidAcidBase

NH4+ + OH-NH3 + H2O

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Conjugate PairsConjugate Pairs

3737

Learning Check!

Label the acid, base, conjugate acid, and Label the acid, base, conjugate acid, and conjugate base in each reaction:conjugate base in each reaction:

HCl + OHHCl + OH-- Cl Cl-- + H + H22OO HCl + OHHCl + OH-- Cl Cl-- + H + H22OO

HH22O + HO + H22SOSO44 HSO HSO44-- + H + H33OO

++ HH22O + HO + H22SOSO44 HSO HSO44-- + H + H33OO

++

3838Acids & Base Acids & Base DefinitionsDefinitions

Lewis acid - a Lewis acid - a substance that substance that accepts an electron accepts an electron pairpair

Lewis base - a Lewis base - a substance that substance that donates an electron donates an electron pairpair

Definition #3 – Lewis Definition #3 – Lewis

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Formation ofFormation of hydronium ion hydronium ion is also an is also an excellent example.excellent example.

Lewis Acids & BasesLewis Acids & Bases

•Electron pair of the new O-H bond Electron pair of the new O-H bond originates on the Lewis base.originates on the Lewis base.

HH

H

BASE

••••••

O—HO—H

H+

ACID

4040The The pH scalepH scale is a way of is a way of expressing the strength expressing the strength of acids and bases. of acids and bases. Instead of using very Instead of using very small numbers, we just small numbers, we just use the NEGATIVE use the NEGATIVE power of 10 on the power of 10 on the Molarity of the HMolarity of the H++ (or (or OHOH--) ion.) ion.

Under 7 = acidUnder 7 = acid 7 = neutral 7 = neutral

Over 7 = baseOver 7 = base

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pH of Common pH of Common SubstancesSubstances

4242Calculating the pH

pH = - log [H+] or [H3O+](Remember that the [ ] mean Molarity)

Example: If [H+] = 1 X 10-10

pH = - log 1 X 10-10

pH = - (- 10)

pH = 10

Example: If [H+] = 1.8 X 10-5

pH = - log 1.8 X 10-5

pH = - (- 4.74)

pH = 4.74

4343

Try These!Try These!

Find the pH of Find the pH of these:these:

1) A 0.015 M 1) A 0.015 M solution of solution of Hydrochloric acidHydrochloric acid

2) A 3.00 X 102) A 3.00 X 10-7-7 M M solution of Nitric solution of Nitric acidacid

4444

Answers!

• 1) 1.82

• 2) 6.51

4545pH calculations – Solving for pH calculations – Solving for H+H+pH calculations – Solving for pH calculations – Solving for H+H+

If the pH of Coke is 3.12, [HIf the pH of Coke is 3.12, [H++] = ???] = ???

Because pH = - log [HBecause pH = - log [H++] then] then

- pH = log [H- pH = log [H++]]

Take antilog (10Take antilog (10xx) of both) of both sides and get sides and get

1010-pH -pH == [H[H++]][H[H++] = 10] = 10-3.12-3.12 = 7.6 x 10 = 7.6 x 10-4-4 M M *** to find antilog on your calculator, look for “Shift” or “2*** to find antilog on your calculator, look for “Shift” or “2nd nd

function” and then the log buttonfunction” and then the log button

4646pH calculations – Solving for pH calculations – Solving for H+H+

• A solution has a pH of 8.5. What is the A solution has a pH of 8.5. What is the Molarity of hydrogen ions in the Molarity of hydrogen ions in the solution?solution?

pH = - log [HpH = - log [H++]]

8.5 = - log [H8.5 = - log [H++]]

-8.5 = log [H-8.5 = log [H++]]

Antilog -8.5 = antilog (log [HAntilog -8.5 = antilog (log [H++])])

1010-8.5-8.5 = [H = [H++]]

3.16 X 103.16 X 10-9-9 = [H = [H++]]

pH = - log [HpH = - log [H++]]

8.5 = - log [H8.5 = - log [H++]]

-8.5 = log [H-8.5 = log [H++]]

Antilog -8.5 = antilog (log [HAntilog -8.5 = antilog (log [H++])])

1010-8.5-8.5 = [H = [H++]]

3.16 X 103.16 X 10-9-9 = [H = [H++]]

4747

More About WaterMore About WaterHH22O can function as both an ACID and a BASE. For O can function as both an ACID and a BASE. For

this reason, water is said to be amphoteric.this reason, water is said to be amphoteric.

In pure water there can beIn pure water there can be AUTOIONIZATIONAUTOIONIZATION

In pure water, the concentration of hydronium In pure water, the concentration of hydronium

and hydroxide have been experimentally and hydroxide have been experimentally

measured as 1.00 x 10measured as 1.00 x 10-7-7 M M

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More About WaterMore About Water

In a In a neutral neutral solution [Hsolution [H33OO++] = [OH] = [OH--] = 1.00 x 10] = 1.00 x 10-7-7 M M

KKw w is the ion product constant of wateris the ion product constant of water

KKww = [H = [H33OO++] [OH] [OH--] = 1.00 x 10] = 1.00 x 10-14-14 at 25 at 25 ooCC

OH-

H3O+

OH-

H3O+

AutoionizationAutoionization

4949

[H+] & [OH-] calculations

• Given a 6.25 x 10-2 M solution of HCl, find [OH-]

Since HCl is a strong acid, it dissociates completely to form H+ and Cl- ions.

» Remember 1 x 10-14 = [H+] [OH-] so,

• 1 x 10-14 = [6.25 x 10-2][OH-]

• [OH-] = 1.60 x 10-13M

5050

Try this!

• The OH-1 concentration of a solution is 8.60 x 10-8. Find the H3O+

5151

Answer

• 1.16 x 10-7

5252pOH

• Since acids and bases are Since acids and bases are opposites, pH and pOH are opposites, pH and pOH are opposites!opposites!

• pOH does not really exist, but it is pOH does not really exist, but it is useful for changing bases to pH.useful for changing bases to pH.

• pOH looks at the perspective of a pOH looks at the perspective of a basebase

pOH = - log [OHpOH = - log [OH--]]Since pH and pOH are on opposite Since pH and pOH are on opposite

ends,ends,pH + pOH = 14pH + pOH = 14

5353

pHpH [H+][H+] [OH-][OH-] pOHpOH

5454

[H[H33OO++], [OH], [OH--] and pH] and pHWhat is the pH of the What is the pH of the

0.0010 M NaOH solution? 0.0010 M NaOH solution?

[OH-] = 0.0010 (or 1.0 X 10[OH-] = 0.0010 (or 1.0 X 10-3-3 M) M)

pOH = - log 0.0010pOH = - log 0.0010

pOH = 3pOH = 3

pH = 14 – 3 = 11pH = 14 – 3 = 11

OR KOR Kww = [H = [H33OO++] [OH] [OH--]]

[H[H3OO++] = 1.0 x 10] = 1.0 x 10-11-11 M M

pH = - log (1.0 x 10pH = - log (1.0 x 10-11-11) = 11.00) = 11.00

5555The pH of rainwater collected in a certain region of the northeastern United States on a particular day was 4.82. What is the H+ ion concentration of the rainwater?

The OH- ion concentration of a blood sample is 2.5 x 10-7 M. What is the pH of the blood?

On your calculator, key in:

2nd then log then ( - ) 4.82 then press ENTER

On your calculator, key in:

2nd then log then ( - ) 4.82 then press ENTER

On your calculator, key in: ( - ) log then 2.5 EE - 7 then press ENTEROn your calculator, key in: ( - ) log then 2.5 EE - 7 then press ENTER

5656

Answers

• 1) 1.51 x 10-5 M

• 2) 6.59

5757

[OH[OH--]]

[H[H++]] pOHpOH

pHpH

1010 -pOH

-pOH

1010 -pH-pH-Log[H

-Log[H++]]

-Log[OH

Log[OH

--]]

14 -

pOH

14 -

pOH

14 -

pH

14 -

pH

1.0

x 10

1.0

x 10-1

4-14

[OH[O

H-- ]]

1.0

x 10

1.0

x 10-1

4-14

[H[H

++ ]]

5858Calculating [H3O+], pH, [OH-], and pOH

Problem 1: A chemist dilutes concentrated hydrochloric acid to make two solutions: (a) 3.0 M and (b) 0.0024 M. Calculate the [H3O+], pH, [OH-], and pOH of the two solutions at 25°C.

Problem 2: A solution has a pH of 3.67. Calculate IN ORDER: [H3O+], [OH-], and pOH. Is this an acid, base, or neutral?

Problem 3: Problem #2 with pH = 8.05?

5959

Answers:

6060

HNO3, HCl, H2SO4 HI, HBr, and HClO4 are strong acids. All other acids are WEAK.

Strong and Weak Strong and Weak Acids/BasesAcids/Bases

The strength of an acid (or base) is determined by the amount of IONIZATION (dissociation).

The strength of an acid (or base) is determined by the amount of IONIZATION (dissociation).

6161


• Generally divide acids and bases into STRONG or Generally divide acids and bases into STRONG or WEAK ones. Strong acids are strong electrolytes WEAK ones. Strong acids are strong electrolytes meaning they dissociate completely in water.meaning they dissociate completely in water.

STRONG ACID:STRONG ACID: HNOHNO3 3 (aq) + H(aq) + H22O (l) --->O (l) --->

HH33OO+ + (aq) + NO(aq) + NO33- - (aq)(aq)

HNOHNO33 is about 100% dissociated in water. is about 100% dissociated in water.

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• Weak acidsWeak acids are much less than 100% ionized in are much less than 100% ionized in

water. Most molecules do NOT dissociate, but water. Most molecules do NOT dissociate, but

stay in tact.stay in tact.

One of the best known is acetic acid = HCOne of the best known is acetic acid = HC22HH33OO22



6363

• Strong Base:Strong Base: 100% dissociated in water. Group IA 100% dissociated in water. Group IA and most of IIA hydroxides are strong bases.and most of IIA hydroxides are strong bases.

NaOH (aq) ---> NaNaOH (aq) ---> Na+ + (aq) + OH(aq) + OH- - (aq)(aq)


Common strong bases Common strong bases include KOH andinclude KOH and Ca(OH)Ca(OH)22..

CaO (lime) + HCaO (lime) + H22O -->O -->

Ca(OH)Ca(OH)22 (slaked lime) (slaked lime)

CaOCaO

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• Weak base:Weak base: less than 100% ionized less than 100% ionized in waterin water

One of the best known weak bases is One of the best known weak bases is ammoniaammonia

NHNH3 3 (aq) + H(aq) + H22O (l) O (l) NH NH44+ + (aq) + OH(aq) + OH- - (aq)(aq)



6565

Weak BasesWeak Bases

6666pH testing

• There are several ways to test pHThere are several ways to test pH

–Blue litmus paper (red = acid)Blue litmus paper (red = acid)

–Red litmus paper (blue = basic)Red litmus paper (blue = basic)

–pH paper (multi-colored)pH paper (multi-colored)

–pH meter (7 is neutral, <7 acid, >7 pH meter (7 is neutral, <7 acid, >7 base)base)

–Universal indicator (multi-colored)Universal indicator (multi-colored)

– Indicators like phenolphthaleinIndicators like phenolphthalein

–Natural indicators like red cabbage, Natural indicators like red cabbage, radishesradishes

6767Paper testing

• Paper tests like litmus paper and pH Paper tests like litmus paper and pH paperpaper

– Put a stirring rod into the solution Put a stirring rod into the solution and stir.and stir.

– Take the stirring rod out, and Take the stirring rod out, and place a drop of the solution from place a drop of the solution from the end of the stirring rod onto a the end of the stirring rod onto a piece of the paperpiece of the paper

– Read and record the color change. Read and record the color change. Note what the color indicates. Note what the color indicates.

– You should only use a small You should only use a small portion of the paper. You can use portion of the paper. You can use one piece of paper for several one piece of paper for several tests.tests.

6868pH indicators

• Indicators are dyes that can be added that will change color in the presence of an acid or base.

• Some indicators only work in a specific range of pH

• Once the drops are added, the sample is ruined

• Some dyes are natural, like radish skin or red cabbage

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• A neutralization reaction is a reaction between an acid and a base.

• ACID + BASE → SALT + WATER

• SALT – is defined as any ionic compound.

• Ex. HBr + KOH → H2O + KBr

NEUTRALIZATION REACTIONSNEUTRALIZATION REACTIONSNEUTRALIZATION REACTIONSNEUTRALIZATION REACTIONS

7070

ACID-BASE REACTIONSACID-BASE REACTIONSTitrationsTitrations

ACID-BASE REACTIONSACID-BASE REACTIONSTitrationsTitrations

• An acid-base titration is a procedure which is used to determine the concentration of an acid or base. A measured volume of an acid or base of known concentration is reacted with a sample to the equivalence point.

• Acid-base titration reactions are neutralization reactions.

7171

• Click here to watch titration movie (no sound)

Setup for titrating an acid with a Setup for titrating an acid with a basebase

7272

TitrationTitrationTitrationTitration

1.1. Add solution from the buret.Add solution from the buret.

2. Reagent (base) reacts with 2. Reagent (base) reacts with compound (acid) in solution compound (acid) in solution in the flask. This is called in the flask. This is called NEUTRALIZATION.NEUTRALIZATION.

3.3. Indicator shows when exact Indicator shows when exact stoichiometric reaction has stoichiometric reaction has occurred. (moles of Acid = occurred. (moles of Acid = moles of Base) This is called moles of Base) This is called the EQUIVALENCE POINT.the EQUIVALENCE POINT.

7373

35.62 mL of NaOH is 35.62 mL of NaOH is

neutralized with 25.2 mL of neutralized with 25.2 mL of

0.0998 M HCl by titration to 0.0998 M HCl by titration to

an equivalence point. What an equivalence point. What

is the concentration of the is the concentration of the

NaOH?NaOH?

Titration problem - accurately Titration problem - accurately determine its concentration.determine its concentration.Titration problem - accurately Titration problem - accurately determine its concentration.determine its concentration.

7474

• Step One: Write the balanced equationNaOH + HCl → H2O + NaCl

• Step Two: Determine the moles of substance using the Molarity and volume of that substance.

– Remember M = moles/Liters so moles = M x L– 0.0252 L x 0.0998 M = 0.00251 moles HCl

• Step Three: – Multiply the moles obtained in step 2 by the mole ratio:

» 0.00251 moles HCl x 1 mole NaOH = 0.00251 mol NaOH

• Step Four: Divide moles in Step 3 by volume of NaOH to find the concentration of NaOH

» 0.00251 mol NaOH = 0.0704 M

Titration solution.Titration solution.Titration solution.Titration solution.

1 mole HCl1 mole HCl

0.03562 L0.03562 L

7575

Example 2 - titration

• Determine the volume of 0.100M HNO3 needed to neutralize 50.00 mL of 0.842M Ca(OH)2.

7676

Answer

• 1.19L

1 chemical equilibrium & acids & bases chemistry i

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