Question

$15\, g$ of ice melts to form water at $ 0{}^\circ C $ . What is the change in entropy?

• A. 18.5
• B. 15
• C. zero
• D. None of these

Answer 1

Answer: (A)

Entropy of a system is a measure of its molecular disorder. The greater the disorder, greater is the entropy. The charge in entropy is given by
$d S =\frac{d Q}{T} $
$=\frac{\text { Heat absorbed }}{\text { Absolute temperature }}$
Also, $d Q=m L$ where, $m$ is mass and $L$ the latent heat.
Given, $ m= 15 \,g , L=80\, cal / g $
$ \therefore d Q=15 \times 80=1200\, cal $
$\therefore d Q=1200 \times 4.2=5040\, J $
Hence, change in entropy is
$d S=\frac{5040}{273}=18.46$
$\approx 18.5$