Question

The internal energy change (in J) when $90 \,g$ of water undergoes complete evaporation at $100^{\circ} C$ is

(Given : $\Delta H _{\text {vap }}$ for water at $373 K =41 kJ / mol$ $\left. R =8.314 JK ^{-1} mol ^{-1}\right)$

Answer 1

$H _{2} O (\ell) \rightleftharpoons H _{2} O ( g ) \,\, 90 gm$ of $H _{2} O$

$\Delta H =\Delta U +\Delta n _{ g } RT $

$ \Rightarrow 5$ moles of $H _{2} O$

$5 \times 41000 J =\Delta U +1 \times 8.314 \times 373 \times 5$

$\Delta U =189494.39$ Joule