Question

Assuming that water vapour is an ideal gas, the internal energy change ($\Delta$U) when 1 mol of water is vapourised at 1 bar pressure and 100⁰C, (Given : Molar enthalpy of vapourization of water at 1 bar and 373K = 41 kJ mol–1 and R = 8.3 J mol–1 K–1) will be :

• A. 37.904 kJ mol–1
• B. 41.00 kJ mol–1
• C. 4.100 kJ mol–1
• D. 3.7904 mol–1

Answer 1

Answer: (A)

37.904 kJ mol–1

Answer 2

$\Delta$U = $\Delta$H – $\Delta$nRT

= 41000 – 1 × 8.314 × 373 = 41000 – 3101.122

= 37898.878 J mol–1 = 37.9 kJ mol–1.