Q105 / 180 · AIIMS 2002 · Physics
The latent heat of vapourisation of water is $2240 \mathrm{~J}$. If the work done in the process of vapourisation of $1 \mathrm{~g}$ is $168 \mathrm{~J}$, then increase in internal energy is
- A$2408 \mathrm{~J}$
- Correct answerB$2072 \mathrm{~J}$
- C$2240 \mathrm{~J}$
- D$1904 \mathrm{~J}$.
Solution
Latent heat of vapourisation of water $(L)=2240 \mathrm{~J}$, mass of the water $(\mathrm{m})=1 \mathrm{~g}$ and work done $(d W)=168 \mathrm{~J}$
From first law of thermodynamics, heat supplied in vapourisation $(d Q)=m L=d U+d W$ or or
$$
\begin{aligned}
& 1 \times 2240=d U+168 \\
& d U=2240-168=2072 \mathrm{~J}
\end{aligned}
$$
(where $d U=$ increase in internal energy).
Hence, the correct answer is option: 'B'.