Chemistry 9701/32 Oct Nov 2009 | Cambridge AS Level Past Papers With Mark Scheme

1. Solubility of Potassium Nitrate: An experiment was conducted to determine how the solubility of potassium nitrate (FB 1) varies with temperature. The mass of FB 1 used, the temperature at which crystals form, and the solubility calculations were recorded.
2. Preparation of Saturated Solutions: Potassium nitrate was dissolved in distilled water, heated until all solids dissolved, and then cooled to record the temperature at which crystals formed. Repeated additions of water lowered the crystallization temperature, and solubility was calculated for each condition.
3. Solubility Data and Plotting: Solubility was calculated using a formula involving the volume of water and mass of FB 1 dissolved. A graph was plotted with solubility against temperature, and the solubility at 42.5°C was deduced from the graph.
4. Explanation of Dissolution Process: The dissolution of potassium nitrate was discussed in terms of whether it is an endothermic or exothermic process, with an emphasis on equilibrium conditions.
5. Sources of Inaccuracy: Two potential sources of inaccuracy in the experiment, apart from poor technique, were suggested, such as inaccurate temperature readings or incomplete dissolution of the solid.
6. Determining Solubility at Higher Temperatures: The solubility of KNO3 at 60°C was discussed, with suggested steps for determining the mass of the solid dissolved and the mass of the solution.
7. Identification of Cations and Anions: The identification of cations and anions in FB 2, FB 3, and FB 4 was carried out using hydrochloric acid, sodium hydroxide, ammonia, and specific reagents like barium chloride and potassium iodide.
8. Observations and Conclusions in Qualitative Analysis: Observations were recorded, and conclusions drawn about the identity of cations and anions present in each of the three solids, using a series of chemical tests and reagents.
9. Tests for Distinguishing Ions: A test was conducted to distinguish between sulfite and sulfate anions, with an explanation of why barium chloride must be added before hydrochloric acid.
10. Reaction with Potassium Iodide: The reaction between FB 2 and potassium iodide was analyzed, and the nature of the reaction was described.