Question

$1g$ of water at atmospheric pressure has volume of $1cc$ and when boiled it becomes $1681cc$ of steam. The heat of vaporisation of water is $540cal/g$. Then the change in internal energy in this process is

• A. 540 cal
• B. 500 cal
• C. 1681 cal
• D. None of the above

Answer 1

Answer: (B)

Given heat, $Q=mL=1\times 540cal$
$=540\times 4.18\times 107J$
Work done, $W=P\Delta V$
$P=1atm=1\times 10^{5}N/m^2=10^{6}dyne/c{m}^2$
$W=10^6\times 1680J$
Change in internal energy
$\Delta U=Q-W$
$=540\times 4.18\times 10^7-10^6\times 1680$
$=\frac{540\times 4.18\times 10^7-10^6\times 1680}{4.18\times 10^7}cal$
$=500cal$