Acids and Bases QP.pdf

AQA Chemistry A-Level - Acids and Bases QP




Q1.
This question is about pH.

Pure water dissociates slightly.

H2O(I) ⇌ H+(aq) + OH–(aq) ΔH = +57 kJ mol–1


The equilibrium constant, Kc =

The ionic product of water, Kw = [H+][OH−]

(a) Explain why [H2O] is not shown in the Kw expression.

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(1)

Table 1 shows how Kw varies with temperature.

Table 1

Temperature /
Kw / mol2 dm–6
°C

10 2.93 × 10−15

20 6.81 × 10−15

25 1.00 × 10−14

30 1.47 × 10−14

50 5.48 × 10−14

(b) Explain why the value of Kw increases as the temperature increases.

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(2)

(c) Give the expression for pH.

Calculate the pH of pure water at 50 °C
Give your answer to 2 decimal places.

Explain why water is neutral at 50 °C



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,AQA Chemistry A-Level - Acids and Bases QP


Expression _________________________________________________

Calculation




pH _______________

Explanation _________________________________________________

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(4)

A pH meter is calibrated using a calibration graph.
To create the calibration, the pH meter is used to measure the pH of separate
solutions, each with a known, accurate pH.

Figure 1 shows the calibration graph.

Figure 1




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, AQA Chemistry A-Level - Acids and Bases QP




(d) Use Figure 1 to give the true pH value when the pH meter reading is 5.6

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(1)

(e) Suggest why the pH probe is washed with distilled water between each of
the calibration measurements.

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(1)

(f) The calibrated pH meter is used to monitor the pH during a titration of
hydrochloric acid with sodium hydroxide.

Explain why the volume of sodium hydroxide solution added between each
pH measurement is smaller as the end point of the titration is approached.

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(1)




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