Learner Resource 4
There are many assumptions that we make when doing pH calculations of acids, bases and buffers.
It is essential that you understand and remember them.
Task 1: Read the assumptions and tick whether they are true or false.
Calculation assumption
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–]
4
When calculating the pH of a buffer we assume that the value given
for the concentration of the salt  [A–].
5
When calculating the pH of a strong acid we assume that the acid has
fully dissociated.
6
When calculating the pH of a weak acid we assume that the acid has
fully dissociated.
7
When calculating the pH of a buffer we assume that [H+]  [A–]
8
When calculating the pH of pure water we assume that Kw = [H+]2
9
When calculating the pH of a strong base we assume that [H+]  [OH–]
10
When calculating the pH of a strong acid, we assume that the
concentration of the acid  [H+].
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
12
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.
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True
False
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Task 2: Write the true calculation assumptions into the correct boxes below according to which
type of calculation they relate to.
Weak acid
Strong acid
Strong base
Water
Buffer
Task 3: Now look at the statements you marked as false in Task 1. First check if there is an
equivalent true statement. If not, modify each statement to make it true and add it to the appropriate
box in Task 2. There are only two to do.
Task 4: Learn these assumptions off by heart.
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