Question

A swimmer coming out from a pool is covered with a film of water weighing about $18 \,g$. The internal energy of vaporization at $100^{\circ}C$ is $(\Delta_{\text{vap}}H$ for water at $373 \,K$ $= 40.66 \,kJ\,mol^{-1})$

• A. $87.56 \,kJ\,mol^{-1}$
• B. $35.56 \,kJ\,mol^{-1}$
• C. $37.56 \,kJ\,mol^{-1}$
• D. $40.5\,kJ\,mol^{-1}$

Answer 1

Answer: (C)

$\Delta_{\text{vap}} U=\Delta_{\text{vap}}H-\Delta\,n_{g}RT$
$H_{2}O_{\left(l\right)} \rightarrow H_{2}O_{\left(g\right)}$
$\Delta \, n_{g}=1-0=1$
assuming steam behaving as an ideal gas, then
$\Delta_{\text{vap}} U=40.66 -1 \times 8.314\times 10^{-3}\times 373$
$=37.56\,kJ\,mol^{-1}$