MCQ
$4.0 \,g$ of a gas occupies $22.4$ litres at $NTP.$ The specific heat capacity of the gas at constant volume is $5.0 \,\,J K^{-1} mol^{-1}$. If the speed of sound in this gas at $NTP$ is $952\, m s^{-1}$, then the heat capacity at constant pressure is .... $J K^{-1} mol^{-1}$ (Take gas constant $R = 8.3 \,\,J K^{-1} mol^{-1}$) 
  • A
    $8.5$
  • $8.0$
  • C
    $7.5$
  • D
    $7.0 $

Answer

Correct option: B.
$8.0$
b
since $4.0 \mathrm{g}$ of a gas occupies $22.4$ litres at $NTP$, so the molecular mass of the gas is

$M=4.0 \mathrm{g} \mathrm{mol}^{-1}$

As the speed of the sound in the gas is

$v=\sqrt{\frac{\gamma R T}{M}}$

where $\gamma$ is the ratio of two specific heats, $R$ is the universal gas constant and $T$ is the temperature of the gas.

$\therefore \gamma=\frac{M v^{2}}{R T}$

Here, $M=4.0 \mathrm{g} \mathrm{mol}^{-1}=4.0 \times 10^{-3} \mathrm{kg} \mathrm{mol}^{-1}$

$v=952 \mathrm{ms}^{-1}, R=8.3 \mathrm{JK}^{-1} \mathrm{mol}^{-1}$ and $T=273 \mathrm{K}(\mathrm{at} \mathrm{NTP})$

$\because \quad \gamma=\frac{\left(4.0 \times 10^{-3} \mathrm{kg} \mathrm{mol}^{-1}\right)\left(952 \mathrm{ms}^{-1}\right)^{2}}{\left(8.3 \mathrm{JK}^{-1} \mathrm{mol}^{-1}\right)(273 \mathrm{K})}=1.6$

By definition, $\gamma=\frac{C_{p}}{C_{v}}$ or $C_{p}=\gamma C_{v}$

But $\gamma=1.6$ and $C_{v}=5.0 \mathrm{JK}^{-1} \mathrm{mol}^{-1}$

$\therefore \quad C_{p}=(1.6)\left(5.0 \mathrm{JK}^{-1} \mathrm{mol}^{-1}\right)=8.0 \mathrm{JK}^{-1} \mathrm{mol}^{-1}$

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