Question

Assuming that water vapour is an ideal gas, the internal energy change (ΔU) when 1mol of water is vapourised at 1 bar pressure and 100°C, (given : molar enthalpy of vapourisation of water at 1 barand 373K = 41kJ mol^–1 and R = 8.3J mol^–1K^–1 ) will be

(a) 41.00kJ mol^–1 

(b) 4.100kJ mol^–1

(c) 3.7904kJ mol^–1

(d) 37.904kJ mol^–1.

Answer 1

(d) : ΔU = ΔH – ΔnRT
= 41000 – 1 × 8.314 × 373
= 41000 – 3101.122
= 37898.878J mol^–1 = 37.9kJ mol^–1.