4.9 Strong and weak acids
5 exam questions worth 9 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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What is a weak acid?
Choose one answer.
Correct Not quite: the answer is An acid that only partially ionises
Mark scheme
C
Model answer
C: a weak acid only partially ionises in water.
Explain the difference between a dilute solution of a strong acid and a concentrated solution of a weak acid.
Mark scheme
- dilute: small amount of acid per volume; concentrated: large amount (1)
- strong acid fully ionises (1)
- weak acid only partially ionises (1)
Model answer
The dilute strong acid has little acid dissolved, but all of it is ionised. The concentrated weak acid has a lot of acid dissolved, but only a small fraction is ionised.
Solution A has pH 1. Solution B has pH 3.
How many times greater is the hydrogen ion concentration in A than in B?
Mark scheme
- 100 times (1)
Model answer
A difference of 2 pH units is a factor of 10 × 10 = 100.
Hydrochloric acid and ethanoic acid of the same concentration are added to magnesium ribbon.
Predict which reacts faster and explain why.
Mark scheme
- hydrochloric acid (1)
- it is fully ionised, so has a higher concentration of \mathrm{H^+} ions / more frequent collisions (1)
Model answer
Hydrochloric acid reacts faster because it is a strong acid, fully ionised, so it has more hydrogen ions for the magnesium to collide with.
Write an equation to show ethanoic acid, CH3COOH, ionising in water.
Mark scheme
- CH3COOH ⇌ \mathrm{H^+} + \mathrm{CH_3COO^-} (1)
- reversible arrow (1)
Model answer
CH3COOH(aq) ⇌ \mathrm{H^+}(aq) + \mathrm{CH_3COO^-}(aq)
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