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2025 O Level Chem P1 Q15 — Stoichiometry / Purity Solution

Stoichiometry / Purity 2025 O Level Pure Chemistry · Paper 1 · Q15

Question

A 20.0 g sample of impure calcium carbonate is added to 0.400 mol of hydrochloric acid. The impurities do not react with hydrochloric acid. After the reaction has finished, there is 0.216 mol of hydrochloric acid remaining. What is the percentage purity of calcium carbonate in the sample?

A23%
B46%
C54%
D92%

Correct answer: B

Video Solution

Explanation

Answer: B. Find the mass of pure \(\text{CaCO}_3\) that reacted.

Moles of HCl reacted = \(0.400 - 0.216 = 0.184\) mol. From \(\text{CaCO}_3 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} + \text{CO}_2\), moles of \(\text{CaCO}_3\) = \(0.184 / 2 = 0.092\) mol.

Mass of pure \(\text{CaCO}_3\) = \(0.092 \times 100 = 9.2\) g.

%purity = \(\frac{9.2}{20.0} \times 100\% = 46\%\) → B.

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