Water of Crystallisation
Cambridge IGCSE Chemistry 0620 · Papers 2 & 4 · 15 Questions
Section A — Definitions
Define the term hydrated.
Define the term anhydrous.
Define the term water of crystallisation.
Cobalt(II) chloride paper turns from blue to pink. Explain what this colour change indicates.
State what is meant by the dot notation in the formula CuSO₄·5H₂O.
Section B — Mr Calculations
Calculate the relative formula mass (Mr) of CuSO₄·5H₂O. [Cu = 64, S = 32, O = 16, H = 1]
Calculate the relative formula mass (Mr) of FeSO₄·7H₂O (iron(II) sulfate heptahydrate). [Fe = 56, S = 32, O = 16, H = 1]
A student dissolves 25.0 g of CuSO₄·5H₂O (Mr = 250) in water. Calculate the number of moles of CuSO₄·5H₂O dissolved.
Calculate the percentage by mass of water in CuSO₄·5H₂O (Mr = 250). [Mr of 5H₂O = 90]
A hydrated salt has the formula MgSO₄·xH₂O. A 12.3 g sample was heated until fully anhydrous. The anhydrous MgSO₄ had a mass of 6.0 g. Find the value of x. [Mr of MgSO₄ = 120, Mr of H₂O = 18]
Section C — Interconversion & Experiments
Describe how you would convert hydrated copper(II) sulfate (CuSO₄·5H₂O) into anhydrous copper(II) sulfate (CuSO₄). Include all observations.
Describe how you would convert anhydrous CuSO₄ back into hydrated CuSO₄·5H₂O. State the observation.
A student heated a hydrated salt and then left the anhydrous product in the open air. Explain what would happen and why.
How can anhydrous cobalt(II) chloride be used to test for the presence of water? State the observation for a positive result.
A student heats 7.45 g of CuSO₄·5H₂O and collects the anhydrous residue. She finds the residue has a mass of 4.79 g. (a) State the colour change observed during heating. [1] (b) Calculate the mass of water lost. [1] (c) Suggest why the student should not heat too strongly. [1]
• Hydrated = contains water of crystallisation (chemically combined)
• Anhydrous = no water of crystallisation present
• Mr of CuSO₄·5H₂O = 160 + 90 = 250
• Hydrated → Anhydrous: Heat gently (blue → white)
• Anhydrous → Hydrated: Add water (white → blue)
• Test for water: Anhydrous CoCl₂ (blue → pink) or anhydrous CuSO₄ (white → blue)
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