ph scale and calculations chapter 14 ph scale we use this scale to measure the strength of an acid...
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pH Scale and Calculations
Chapter 14
pH Scale
• We use this scale to measure the strength of an acid or base.
• pH is defined as the –log[H+]
• pH can use the concentration of hydronium ions or hydrogen ions.
pH Scale
Acid Base
0
7
14
Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 515
pH of Common Substances
Timberlake, Chemistry 7th Edition, page 335
LecturePLUS Timberlake 5
Ionization of Water
Occasionally, in water, a H+ is transferred between H2O molecules
. . . . . . . .H:O: + :O:H H:O:H + + :O:H-
. . . . . . . . H H H
water molecules hydronium hydroxide ion (+) ion (-)
6
Pure Water is Neutral
Pure water contains small, but equal amounts of ions: H3O+ and OH-
H2O + H2O H3O+ + OH-
hydronium hydroxide
ion ion
1 x 10-7 M 1 x 10-7 MH3O+ OH-
7
Ion Product of Water Kw
Kw = [ H3O+ ] [ OH- ]
= [ 1 x 10-7 ][ 1 x 10-7 ]
= 1 x 10-14
pH of Common Substance
14 1 x 10-14 1 x 10-0 0 13 1 x 10-13 1 x 10-1 1 12 1 x 10-12 1 x 10-2 2 11 1 x 10-11 1 x 10-3 3 10 1 x 10-10 1 x 10-4 4 9 1 x 10-9 1 x 10-5 5 8 1 x 10-8 1 x 10-6 6
6 1 x 10-6 1 x 10-8 8 5 1 x 10-5 1 x 10-9 9 4 1 x 10-4 1 x 10-10 10 3 1 x 10-3 1 x 10-11 11 2 1 x 10-2 1 x 10-12 12 1 1 x 10-1 1 x 10-13 13 0 1 x 100 1 x 10-14 14
NaOH, 0.1 MHousehold bleachHousehold ammonia
Lime waterMilk of magnesia
Borax
Baking sodaEgg white, seawaterHuman blood, tearsMilkSalivaRain
Black coffeeBananaTomatoesWineCola, vinegarLemon juice
Gastric juice
Mor
e ba
sic
Mor
e ac
idic
pH [H1+] [OH1-] pOH
7 1 x 10-7 1 x 10-7 7
Acid – Base Concentrations
pH = 3
pH = 7
pH = 11
OH-
H3O+OH-
OH-H3O+
H3O+
[H3O+] = [OH-] [H3O+] > [OH-] [H3O+] < [OH-]
acidicsolution
neutralsolution
basicsolution
co
nc
en
trat
ion
(m
ole
s/L
)
10-14
10-7
10-1
Timberlake, Chemistry 7th Edition, page 332
pH
pH = -log [H+]
Kelter, Carr, Scott, Chemistry A World of Choices 1999, page 285
Self-Ionization Of Water
• Even the purest of water conducts electricity. This is due to the fact that water self-ionizes, that is, it creates a small amount of H3O+ and OH-.
H2O + H2O H3O+ + OH-
Kw = [H3O+][OH-]
• Kw - ion product of water
Kw = 1.0 x 10-14 at 25 oC
• This equilibrium constant is very important because it applies to all aqueous solutions - acids, bases, salts, and non-electrolytes - not just to pure water.
OH
H
OH
HO
Self ionization reaction of water:
+O
HH
H
OHOHOH2 32
C)25(at10][OH]O[HK -143w
[OH-]
Kw]O[H3
H
+
+-
pH and pOH
• pH = - log[H3O+] [H3O+] = 10-pH
pOH = - log[OH-] [OH-] = 10-pOH
• pKw = pH + pOH = 14.00
• neutral solution: [H3O+] = [OH-] = 10 –7 M pH = 7.0
acidic solution: [H3O+] > 10-7 M pH < 7.0
basic solution: [H3O+] < 10-7 M pH > 7.0
LecturePLUS Timberlake 14
Learning Check pH
A. The [H3O+] of tomato juice is 1 x 10-4 M.
What is the pH of the solution?
1) - 4 2) 4 3) 8
B. The [OH-] of an ammonia solution is
1 x 10-3 M. What is the pH of the solution?
1) 3 2) 11 3) -11
LecturePLUS Timberlake 15
Solution pH4
A. pH = - log [ 1 x 10-4] = -(- 4) = 4
B. [H3O+] = 1 x 10-11
pH = - log [ 1 x 10- 11] = -(- 11) = 11
LecturePLUS Timberlake 16
Learning Check pH5
The pH of a soap is 10. What is the [H3O+] of the soap
solution?
1) 1 x 10 - 4 M
2) 1 x 1010 M
3) 1 x 10 - 10 M
LecturePLUS Timberlake 17
Solution pH5
The pH of a soap is 10. What is the [H3O+]
of the soap solution?
[H3O+] = 1 x 10-pH M
= 1 x 10-10 M
18
pH on the Calculator
[H3O+] is 4.5 x 10-6 M
pH = -log(4.5 x10-6 )
= 5.35
LecturePLUS Timberlake 19
Learning Check pH6
A soap solution has a [H3O+] = 2 x 10-8 M. What is the pH of
the solution?
1) 8
2) 7.7
3) 6
20
Solution pH6
A soap solution has a [H3O+] = 2.0 x 10-
8 M. What is the pH of the solution?
B) = 7.7
Calculating pH from pOH
• The concentration of [OH–] in a cleaning product was found to be 10–3 mol L–1.
• Find the pH of the solution.
Practise
• (a) A 0.01 mol L–1 solution of HNO3 is prepared. Calculate the pH of the
• solution.
• (b) A 0.005 mol L–1 solution of H2SO4 is prepared. Assuming complete
• ionisation, calculate the pH of the solution.
Sample Problem: Calculating the pH of a dilution
• 50 mL of a solution of HCl of pH 2 is diluted so that the new pH is 5. What
• volume of water was added?