At STP, 4.0 g of helium occupies a volume of 22400 cm^3 . Given...

Given mass of helium, $m = 4.0 \text{ g} = 4.0 \times 10^{-3} \text{ kg}$

Volume at STP, $V = 22400 \text{ cm}^3 = 22.4 \times 10^{-3} \text{ m}^3$

Density of helium, $\rho = \dfrac{m}{V} = \dfrac{4.0 \times 10^{-3}}{22.4 \times 10^{-3}} = \dfrac{1}{5.6} \text{ kg m}^{-3}$

Molar heat capacity at constant pressure, $C_p = 5.0 \text{ cal K}^{-1} \text{ mol}^{-1}$

Universal gas constant, $R \approx 2.0 \text{ cal K}^{-1} \text{ mol}^{-1}$

Molar heat capacity at constant volume, $C_v = C_p - R = 5.0 - 2.0 = 3.0 \text{ cal K}^{-1} \text{ mol}^{-1}$

Ratio of specific heats, $\gamma = \dfrac{C_p}{C_v} = \dfrac{5}{3}$

Pressure at STP, $P = 1.013 \times 10^5 \text{ N m}^{-2}$

The speed of sound in a gas is given by Laplace's formula:

$v = \sqrt{\dfrac{\gamma P}{\rho}}$

Substituting the values:

$v = \sqrt{\dfrac{5}{3} \times 1.013 \times 10^5 \times 5.6}$

$v = \sqrt{\dfrac{28 \times 1.013 \times 10^5}{3}}$

$v = \sqrt{9.454 \times 10^5} \approx 972 \text{ m s}^{-1}$