First slide

Travelling wave

Question

The ratio of the speed of sound in nitrogen gas to that in helium gas, at 300 K is

Moderate

A

$\sqrt{2 / 7}$
B

$\sqrt{1 / 7}$
C

$\sqrt{3} / 5$
D

$\sqrt{6} / 5$

Solution

Velocity of sound in gas $v = \sqrt{\frac{γRT}{M}} \Rightarrow v \propto \sqrt{\frac{γT}{M}}$
$\Rightarrow \frac{v_{N_{2}}}{v_{He}} = \sqrt{\frac{γ_{N_{2}}}{γ_{He}} \times \frac{M_{He}}{M_{H_{2}}}} = \sqrt{\frac{\frac{7}{5} R \times 4}{\frac{5}{3} R \times 28}} = \frac{\sqrt{3}}{5}$

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