# A student reacts excess zinc metal with 25.0 cm³ of 0.500 mol dm⁻³ hydrochloric acid. After filtering off the unreacted zinc and evaporating the solution, the student obtains 0.76 g of zinc chloride. Explain what is meant by the terms limiting reagent and percentage yield in this context, and explain why the percentage yield of zinc chloride is less than 100%.

> IB DP Chemistry Higher Level (2023 syllabus) — R2.1 How much? The amount of chemical change · Explain · 4 marks

> The reaction is: Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)

## Mark scheme (4 marks)

1. The limiting reagent is HCl because it is present in the smaller stoichiometric amount and is completely consumed, thereby determining the maximum amount of product that can form.
2. Percentage yield is the ratio of the actual (experimental) yield to the theoretical (maximum) yield, expressed as a percentage: (actual yield / theoretical yield) × 100.
3. The theoretical yield of ZnCl₂ is 0.0125 mol (from 0.0125 mol HCl), giving a theoretical mass of approximately 1.70 g; the percentage yield is therefore approximately 45%.
4. The percentage yield is less than 100% because of losses during the experimental procedure (e.g. product remains dissolved in solution or adheres to the filter/evaporating dish) or because the reaction may not have reached completion.

## Key terms

- [limiting reagent](https://www.gradenine.co.uk/glossary/limiting-reagent)
- [percentage yield](https://www.gradenine.co.uk/glossary/percentage-yield)

## Related

- [Revision notes for IB DP Chemistry Higher 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/a-student-reacts-excess-zinc-metal-fd1c6b4a) · Published by Druglandscape Ltd.