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Ex1) Calculating pH for a
Weak Acid Solution
Ex1) Find the pH of a 2.5 M solution of HF.
H+(aq) + F-(aq)
HF(aq)
(Ka = 7.2 x 10-4)
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Acids and Bases 14.5
[H+]
[HF]
pH for Weak Acids
Polyprotic Acids
[F-]
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Step 1. Find [H+] by using an ICE chart.
Ex1) Calculating pH for a
Weak Acid Solution (cont.)
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Ex1) Calculating pH for a
Weak Acid Solution (cont.)
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Step 2. Find the pH using your calculated
value for [H+].
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Ex2) Calculating pH for a
Weak Acid Solution
Ex2) Calculating pH for a
Weak Acid Solution (cont.)
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Ex2) Find the pH of a 1.5 M HCN solution.
H+(aq) + CN-(aq) (Ka = 4.9 x 10-10)
HCN(aq)
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Step 1. Find [H+] with an ICE chart
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[HCN]
[H+]
[CN-]
Step 2. Find the pH with [H+]
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Ex3) Ka and pH with % (cont.)
Ex3) Ka and pH with % Dissociation
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[C2H3O2-]
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[H+]
[HC2H3O2]
H+ + C2H3O2-
HC2H3O2
Ex 3) Acetic acid, HC2H3O2 , experiences 0.767%
dissociation in a 0.300 M solution at 25oC. Find Ka
for acetic acid at 25oC and the pH of the solution.
Step 1. Find Ka
Step 2. Find pH
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Polyprotic Acids
H2CO3(aq)
H+(aq) + HCO3-(aq) (Ka1 = 4.3 x 10-7)
HCO3-(aq)
H+(aq) + CO32-(aq) (Ka2 = 5.6 x 10-11)
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Carbonic Acid
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e.g. H2SO4 and H2CO3
Sulfuric acid
Polyprotic acids have a different Ka value for
each possible dissociation.
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Polyprotic acids can donate more than one H+
in a solution.
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Polyprotic Acids
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Polyprotic Acids
(Ka1 = 4.3 x 10-7) >> (Ka2 = 5.6 x 10-11)
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Always use Ka1 to calculate [H+] and pH.
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• Most of the H+ ions come from the first
dissociation.
• The H+ ions from the first dissociation drive the
equilibrium(s) for the other dissociations to the left.
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