A Level Chemistry OCR Practice Exam

Determining the amount of water of crystallisation in a sample is important for assessing its purity because the presence of water of crystallisation can indicate how well the sample has been formed and how consistent its chemical structure is with the expected formula. If a sample contains an abnormal amount of water of crystallisation compared to its expected value, it may suggest contamination or the presence of impurities. This assessment helps establish the reliability of the sample for further applications or research.

The other options have relevance in their own contexts but do not specifically relate to the significance of measuring water of crystallisation. For instance, while water of crystallisation can influence solubility, the primary concern when analyzing a sample is usually regarding its composition and purity. Similarly, while knowing water content can contribute to understanding molecular weight, it is not the main focus of purity analysis. Thermal stability is more related to the bond energy within the compound and its behavior under heat rather than the specific aspect of crystallised water.