You have 3.51g of hydrated zinc sulphate. You heat up the zinc sulphate until all the water has evaporated from it. The weight after heating is 1.97g. Find how many H2O molecules per zinc sulphate molecule there are in the hydrated form of it.
- Write down the equation: ZnSO4xH2O --> ZnSO4 + xH2O 2. Calculate the masses: 3.51g --> 1.97g + 1.54g 3. Calculate the number of moles, using the molar mass of water and zinc sulphate: 1.97g = 0.0122mol and 1.54g = 0.085mol 4. Understand that the ratio of the molecules in the equation can be deduced from the ratio of moles. 5. Calculate the ratio from the values 0.0122 and 0.085 --> 1:7 6. Complete the molecular formulae: ZnSO4*7H2O --> ZnSO4 + 7H2O 7. The answer is: A hydrated molecule of zinc sulphate contains 7 molecules of water. I would use a diagram to initially explain the concept of water molecules being trapped in a solid salt. Subsequently I would take the student through the steps of understanding that the differences in weight need to be converted into moles and that this provides us with the required information to deduce how many water molecules are trapped in the hydrated salt.
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