# Explain why the pH of a solution of carbonic acid (H₂CO₃) is higher than the pH of a hydrochloric acid solution of the same concentration.

> IB DP Chemistry Standard Level (2023 syllabus) — R3.1 Proton transfer reactions · Explain · 4 marks

> Carbonic acid (H₂CO₃) and hydrochloric acid (HCl) are both present in water at a concentration of 0.10 mol dm⁻³.

## Mark scheme (4 marks)

1. HCl is a strong acid that dissociates completely/fully in water, producing the maximum possible concentration of H⁺ ions.
2. H₂CO₃ is a weak acid that only partially dissociates/ionises in water, so the equilibrium lies to the left.
3. At the same concentration, the H⁺ (or H₃O⁺) ion concentration in the H₂CO₃ solution is lower than in the HCl solution.
4. A lower [H⁺] corresponds to a higher pH, so the H₂CO₃ solution has a higher pH than the HCl solution.

## Key terms

- [pH](https://www.gradenine.co.uk/glossary/ph)

## Related

- [Revision notes for IB DP Chemistry Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-ph-of-a-55e0e8a7) · Published by Druglandscape Ltd.