Unit mass of a liquid with volume ${V_1}$ is completely changed into a gas of volume ${V_2}$ at a constant external pressure $P$ and temperature $T.$ If the latent heat of evaporation for the given mass is $L,$ then the increase in the internal energy of the system is
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(c) $\Delta Q = \Delta V + P\Delta V$$\Rightarrow mL = \Delta U + P(V_2 -V_1)$
$\Rightarrow$ $\Delta U = L -P (V_2 -V_1)\; ( m = 1)$
$\Rightarrow$ $\Delta U = L -P (V_2 -V_1)\; ( m = 1)$
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