Citric acid is a weak acid found in citrus fruits. Hydrochloric acid is a strong acid. Both acids can be prepared as solutions with the same concentration. Explain why a solution of citric acid at a given concentration has a higher pH than a solution of hydrochloric acid at the same concentration.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Citric acid (H₃C₆H₅O₇) is a weak acid naturally present in lemons and other citrus fruits. Hydrochloric acid (HCl) is a strong acid commonly used in industry and laboratories.
Model answer (5 marks)
Hydrochloric acid is a strong acid and is completely dissociated in aqueous solution, giving a concentration of H⁺ equal to the acid concentration.
Citric acid is a weak acid and only partially dissociates, so the concentration of H⁺ produced is less than the acid concentration.
Because the H⁺ concentration is lower in the citric acid solution, the pH, which is a measure of H⁺ concentration, is higher for citric acid than for hydrochloric acid at the same concentration.
Citric acid is a weak acid and only partially dissociates, so the concentration of H⁺ produced is less than the acid concentration.
Because the H⁺ concentration is lower in the citric acid solution, the pH, which is a measure of H⁺ concentration, is higher for citric acid than for hydrochloric acid at the same concentration.
Examiner tips
- State that HCl is fully dissociated, citric acid only partially.
- Show that lower [H⁺] gives higher pH.
- Link the points in a logical sequence.
Common mistakes
- Confusing pH with concentration, e.g. saying higher concentration gives higher pH.
- Forgetting to mention that citric acid is only partially dissociated.
- Using incorrect terminology such as ‘weak base’ instead of ‘weak acid’.
Mark scheme (5 marks)
- Hydrochloric acid is fully/completely ionised (dissociated) in aqueous solution
- Citric acid is only partially ionised (dissociated) in aqueous solution
- Therefore the concentration of hydrogen ions (H⁺) is lower in the citric acid solution than in the hydrochloric acid solution at the same concentration
- pH is a measure of hydrogen ion concentration / higher [H⁺] gives lower pH
- Because citric acid has a lower [H⁺] than HCl at the same concentration, citric acid has a higher pH
Key terms in this question
strong acid · weak acid · pH
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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