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JAMB Chemistry 1987 Question 26

Question 26

Energy Changes

5 g of ammonium trioxonitrate (V) on dissolution in water cooled its surrounding water and container by 1.6 kJ. What is the heat of solution of NH4NO3? [N = 14, O = 16, H = 1]

  1. A.+51.4 kJ mol−1
  2. B.+25.6 kJ mol−1
  3. C.+12.9 kJ mol−1
  4. D.−6.4 kJ mol−1
Show the answer and explanation

Answer: B (+25.6 kJ mol−1)

Explanation

M(NH4NO3) = 80, so 5 g is 0.0625 mol. Heat absorbed per mole = 1.6/0.0625 = 25.6 kJ; it cools the water, so it is endothermic (+25.6 kJ mol−1).