Reaction with metals

2 marks · Hard

Butanoic acid and hydrochloric acid have the same concentration. Explain why the pH of butanoic acid is higher than that of hydrochloric acid.

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Butanoic acid is a weak acid so only partially ionises in water, producing fewer hydrogen ions than hydrochloric acid, which is a strong acid and ionises completely.

Also accept: Butanoic acid only partially ionises in water, so its solution has fewer H+ ions than hydrochloric acid at the same concentration.; Because butanoic acid is a weak acid and hydrochloric acid is strong, butanoic acid produces fewer hydrogen ions so the pH is higher.

Common mistakes: States only 'butanoic acid is weaker' without explanation.; Says 'because butanoic acid is less concentrated'.

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Marking points

M1 · 1 mark · Recognises that butanoic acid is a weak acid (or that hydrochloric is strong) and so only partially ionises in water.

A1 · 1 mark · States that this leads to fewer hydrogen ions (or a higher pH) compared to hydrochloric acid.

Worked solution

Butanoic acid is a weak acid and only partially ionises in water.

This means there are fewer hydrogen ions (H+) in the solution compared to hydrochloric acid, which ionises completely. Therefore, butanoic acid has a higher pH.