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JAMB Chemistry 1998 Question 3

Question 3

Mole Concept and Stoichiometry

8.0 g of an element X reacted with an excess of copper (II) tetraoxosulphate (VI) solution to deposit 21.3g of copper. The correct equation for the reaction is [Cu = 64].

  1. A.X(s) + CuSO4(aq) → Cu(s) + XSO4(aq)
  2. B.X(s) + 2CuSO4(aq) → 2Cu(s) + X(SO4)2(aq)
  3. C.2X(s) + CuSO4(aq) → Cu(s) + X2SO4(aq)
  4. D.2X(s) + 3CuSO4(aq) → 3Cu(s) + X2(SO4)3(aq)
Show the answer and explanation

Answer: A (X(s) + CuSO4(aq) → Cu(s) + XSO4(aq))

Explanation

21.3 g of Cu is 21.3/64 ≈ 0.333 mol. With a 1 : 1 ratio, 8.0 g of X is also 0.333 mol, so X has relative atomic mass 24, which is magnesium, a real divalent metal that displaces copper. The other ratios give masses that match no suitable metal.