OCR a Level Chemistry a Delivery Guide (Learner Resource) - Teacher Sheet 1 -Ph Buffers

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–] X

2 When calculating the pH of a strong base we assume that [H+] = [OH–] X

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 X for the concentration of the salt  [A–].

5 When calculating the pH of a strong acid we assume that the acid has X fully dissociated.

6 When calculating the pH of a weak acid we assume that the acid has X fully dissociated.

7 When calculating the pH of a buffer we assume that [H+]  [A–] X

+ 2 8 When calculating the pH of pure water we assume that Kw = [H ] 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 X 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 – X amount of base.

12 In buffers made from an excess of weak acid we assume that [A–]  X [OH–], i.e. calculated from the amount of base given in the question.

Version 1 1 © OCR 2016 pH and Buffers 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–]

+ 2 When calculating the pH of pure water we assume that Kw = [H ]

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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