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JAMB Chemistry 1983 Question 6

Question 6

Mole Concept and Stoichiometry

Some copper (II) sulphate pentahydrate (CuSO4.5H2O), was heated at 120°C with the following results: Wt of crucible = 10.00 g; Wt of crucible + CuSO4.5H2O = 14.98g; Wt of crucible + residue = 13.54 g. How many molecules of water of crystallization were lost? [H = 1, Cu = 63.5, O = 16, S = 32]

  1. A.1
  2. B.2
  3. C.3
  4. D.4
  5. E.5
Show the answer and explanation

Answer: D (4)

Explanation

Hydrate = 4.98 g = 4.98/249.5 ≈ 0.020 mol. Water lost = 14.98 − 13.54 = 1.44 g = 0.080 mol. 0.080/0.020 = 4 molecules of water per formula unit.