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26/09/2021 11:56 am
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At 25°C, when 1 mole of MgSO4 was dissolved in water, the heat evolved was found to be 91.2 kJ. One mole of MgSO4.7H2O on dissolution gives a solution of the same composition accompanied by an absorption of 13.8 kJ. The enthalpy of hydration, i.e., ΔHh for the reaction MgSO4(s) + 7H2O(l) → MgSO4.7H2O(s) is:
(a) -105 kJ/mol
(b) -77.4 kJ/mol
(c) 105 kJ/mol
(d) None of these
1 Answer
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26/09/2021 12:02 pm
Correct answer: (a) -105 kJ/mol
Explanation:
Given that
MgSO4(s) + 7H2O(l) → MgSO4.nH2O(s)
ΔrH1 = – 91.2 kJ/mol ...(i)
MgSO4(s) + 7H2O(s) + (n - 7)H2O → MgSO4.(nH2O)
ΔrH2 = 13.8 kJ/mol ...(ii)
or ΔHhyd = ΔrH1 – ΔrH2 equation (i) – (ii)
= -91.2 kJ/mol - 13.8 kJ/mol
= -105 kJ/mol