Ch. 19 – Acids & Bases
II. pH
(p. 644 – 658)
A. Ionization of Water
H 2O + H 2 O
H3
+
O
Self-Ionization of Water
+
OH
A. Ionization of Water
Kw = [H3O+][OH-] = 1.0  10-14
Ion Product Constant for Water
• The ion production of water, Kw = [H3O+][OH–]
• Pure water contains equal concentrations of H+
and OH– ions, so [H3O+] = [OH–]
• For all aqueous solutions, the product of the
hydrogen-ion concentration and the hydroxide-ion
concentration equals 1.0 x 10-14
A. Ionization of Water
 Find
the hydroxide ion concentration of
3.0  10-2 M HCl.
HCl
→
H+
+
Cl3.0  10-2M
3.0  10-2M
[H3O+][OH-] = 1.0  10-14
[3.0  10-2][OH-] = 1.0  10-14
[OH-] = 3.3  10-13 M
A. Ionization of Water
 Find
the hydronium ion concentration of
1.4  10-3 M Ca(OH)2.
Ca(OH)2 →
Ca2+
+
2 OH1.4  10-3M
2.8  10-3M
[H3O+][OH-] = 1.0  10-14
[H3O+][2.8  10-3] = 1.0  10-14
[H3O+] = 3.6  10-12 M
B. pH Scale
14
0
7
INCREASING
ACIDITY
NEUTRAL
pH =
INCREASING
BASICITY
+
-log[H3O ]
pouvoir hydrogène (Fr.)
“hydrogen power”
B. pH Scale
pH of Common Substances
B. pH Scale
pH =
+
-log[H3O ]
pOH =
-log[OH ]
pH + pOH = 14
B. pH Scale
 What
is the pH of 0.050 M HNO3?
pH = -log[H3O+]
pH = -log[0.050]
pH = 1.30
Acidic or basic? Acidic
B. pH Scale
 What
is the pH of 0.050 M Ba(OH)2?
[OH-] = 0.100 M
pOH = -log[OH-]
pOH = -log[0.100]
pOH = 1.00
pH = 13.00
Acidic or basic?
Basic
B. pH Scale
 What
is the molarity of HBr in a solution
that has a pOH of 9.60?
pH + pOH = 14
pH = -log[H3O+]
pH + 9.60 = 14
4.40 = -log[H3O+]
pH = 4.40
-4.40 = log[H3O+]
Acidic
[H3O+] = 4.0  10-5 M HBr
C. pH Worksheet #6
A swimming pool has a volume of one million liters. How many grams of HCl
would need to be added to that swimming pool to bring the pH down from
7.0000 to 4.0000? (Assume the volume of the HCl is negligible)
7 = -log[H+]
-7 = log[H+]
[H+] = 1 x 10-7 M
-4 = log[H+]
[H+] = 1 x 10-4 M
1,000,000L
Sol’n
1x10-7
mol H+
1L
soln
1,000,000L
Sol’n
1x10-4
mol H+
1L
soln
= 0.1 mol
H+
= 100 mol
H+
C. pH Worksheet #6
A swimming pool has a volume of one million liters. How many grams of
HCl would need to be added to that swimming pool to bring the pH down
from 7.0000 to 4.0000? (Assume the volume of the HCl is negligible)
100 mol H+ – 0.1 mol H+
= 99.9 mol HCl
99.9 mol
HCl
36.46 g HCl
1 mol
HCl
= 3642 g HCl
D. pH Sig Figs
 For
the pH, the number of sig figs is
shown by the # of decimal places
• [H+] = 2.26 x 10-4 M => pH = 3.646
 For the molarity from the pH, check
decimal places in the pH
• pH = 4.25 => 5.6 x 10-5 M