# Explain why the mass of carbon dioxide produced when excess hydrochloric acid reacts with calcium carbonate can be used to determine the percentage yield of the reaction, and outline ONE limitation of using mass loss to measure the amount of carbon dioxide produced.

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

> A student reacts a weighed sample of impure calcium carbonate with excess hydrochloric acid in an open conical flask placed on a balance. The balanced equation for the reaction is: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

## Mark scheme (4 marks)

1. The moles of CO₂ produced are stoichiometrically equal to the moles of CaCO₃ that reacted (1:1 molar ratio from the equation), so the actual yield of CaCO₃ converted can be calculated from the mass of CO₂ lost.
2. The theoretical yield is calculated from the moles of pure CaCO₃ in the sample (using molar mass 100 g mol⁻¹), and percentage yield is actual yield divided by theoretical yield multiplied by 100.
3. A limitation is that CO₂ may escape before the reaction is complete or before the balance reading is taken, causing the recorded mass loss to be greater than the true amount produced during the timed measurement, leading to an overestimate.
4. Another limitation is that water vapour (or HCl vapour / spray) may also be lost from the open flask, contributing to the recorded mass loss and causing the mass of CO₂ to be overestimated.

## Key terms

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

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-mass-of-carbon-09b0dde5) · Published by Druglandscape Ltd.