A Level Chemistry OCR Practice Exam

What is the empirical formula of a hydrated compound with 5 water molecules of crystallisation?

1. MH5O

2. M(H2O)5

3. M·5H2O

4. MH10

The correct answer is: M·5H2O

The empirical formula of a hydrated compound represents the simplest whole-number ratio of the elements in that compound. In the case of a hydrated compound with 5 water molecules of crystallization, the correct representation accounts for both the metal ion (M) and the associated water molecules. When you see the notation M·5H2O, it clearly indicates that for every one metal ion (M), there are five water molecules (5H2O) associated with it. This format effectively expresses the water of crystallization in a way that underscores the stoichiometric relationship between the metal and the water. The other choices reflect different ways of expressing the relationship. For instance, MH5O suggests an incorrect molecule composition without properly conveying the association of water. M(H2O)5, while representing that there are 5 water molecules, does not clearly highlight the relationship with the metal. Meanwhile, MH10 implies that the compound consists of one metal and 10 hydrogen atoms, which does not adequately describe the presence of the water molecules. Thus, M·5H2O is the most appropriate representation for the compound, emphasizing both the metal and its five water molecules in a clear and concise manner.