Chemistry 9701/31 Oct Nov 2016 | Cambridge AS Level Past Papers With Mark Scheme

1. Determining Percentage Purity of Calcium Carbonate:
- Method 1: Measuring the volume of carbon dioxide gas produced when calcium carbonate reacts with excess hydrochloric acid.
- The mass of calcium carbonate and the volume of gas are used to calculate the purity of the sample.
- Impurities in the calcium carbonate do not react with the acid, so they are excluded from the calculation.
- Improvement suggestion: Identifying changes to the method that would lead to a better measurement of carbon dioxide.

2. Titration of Hydrochloric Acid:
- After the reaction in Method 1, the remaining hydrochloric acid is titrated with sodium hydroxide.
- The volume of sodium hydroxide required for neutralization is used to calculate the amount of hydrochloric acid left and thus the amount that reacted with the calcium carbonate.

3. Qualitative Analysis:
- Conducting tests on solids containing cations and anions (FA 5 and FA 6) to identify their components using methods like heating with sulfuric acid, testing with sodium hydroxide and ammonia, and observing any precipitate or gas formation.
- The document includes details on how to test and identify ions such as chloride, sulfate, and ammonia ions, with specific reactions and expected observations.

4. Calculations:
- Using titration data and gas volume measurements to determine the moles of substances involved and to calculate the purity of calcium carbonate in the sample.

5. Error Analysis:
- Discussing how errors, such as loss of carbon dioxide during the reaction, can affect the results in both methods and suggesting ways to minimize these errors.