Teacher Resource 4
Expected answers to Learner Resource 4
Task 1
Calculation assumption
True
False
1
When calculating the pH of a weak acid we assume that [H+]  [A–]
2
When calculating the pH of a strong base we assume that [H+] = [OH–]
3
When calculating the pH of pure water we assume that [H+] = [OH–]
X
4
When calculating the pH of a buffer we assume that the value given
for the concentration of the salt  [A–].
X
5
When calculating the pH of a strong acid we assume that the acid has
fully dissociated.
X
6
When calculating the pH of a weak acid we assume that the acid has
fully dissociated.
X
7
When calculating the pH of a buffer we assume that [H+]  [A–]
X
8
When calculating the pH of pure water we assume that Kw = [H+]2
X
9
When calculating the pH of a strong base we assume that [H+]  [OH–]
X
10
When calculating the pH of a strong acid, we assume that the
concentration of the acid  [H+].
X
11
In buffers made from an excess of weak acid we assume that [HA] 
[excess acid calculated], i.e. calculated using amount of acid –
amount of base.
X
In buffers made from an excess of weak acid we assume that [A–] 
[OH–], i.e. calculated from the amount of base given in the question.
X
12
Version 1
pH and Buffers
1
X
X
© OCR 2016
Task 2 and Task 3
Statements in italics are rewritten from false statements.
Weak acid
When calculating the pH of a weak acid we assume that [H+]  [A–]
When calculating the pH of a weak acid we assume the acid has not fully
dissociated.
Strong acid
When calculating the pH of a strong acid we assume that the acid has fully
dissociated.
When calculating the pH of a strong acid, we assume that the concentration of the
acid  [H+].
Strong base
When calculating the pH of a strong base we assume that [H+]  [OH–]
Water
When calculating the pH of pure water we assume that [H+] = [OH–]
When calculating the pH of pure water we assume that Kw = [H+]2
Buffer
When calculating the pH of a buffer we assume that the value given for the
concentration of the salt  [A–].
In buffers made from an excess of weak acid we assume that [HA]  [excess acid
calculated], i.e. calculated using amount of acid – amount of base.
In buffers made from an excess of weak acid we assume that [A–]  [OH–], i.e.
calculated from the amount of base given in the question.
When calculating the pH of a buffer we do not assume that [H+]  [A–]
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Version 1
pH and Buffers
2
© OCR 2016