Question

A piston exerting a pressure of 1.0 atmosphere rests on the surface of water at ${100}^{\circ }C$. The pressure is reduced to smaller extent and as a result 10 g of water evaporates and absorbs 22.2 kJ of heat. The change in internal energy is:

• A. 18.24 kJ
• B. 20.477 kJ
• C. 22.05 kJ
• D. 23.923 kJ

Answer 1

The correct option is B 20.477 kJ

The relationship between the enthalpy change and the change in the internal energy is
$\Delta H=\Delta U+P\Delta V=\Delta U+\Delta n_{g}RT$
$\Delta U=\Delta H-\Delta n_{g}RT$
Substitute values in the above expression.
$\Delta U=22.2-\frac{10}{18}\times 8.314\times {10}^{-3}\times 373$
$=20.477kJ$
The change in internal energy is 20.477 kJ