As seen from equation (1-28), the velocity of sound in a gas depends on g, the ratio of the principle specific heats of the gas. This, in turn, depends on the atomicity of the gas. An approximate equation connecting g and the degrees of freedom is
Where n is the number of degrees of freedom. For a monatomic gas n= 3, for a diatomic gas n = 5 and for a triatomic gas n = 7. Hence, the value of g for these gases will be about 1.67, 1.4 and 1.3 respectively.
If two gases having the same value of g are considered, like, hydrogen and oxygen, then
When their pressures are same, oxygen is nearly 16 times heavier than hydrogen. The velocity of sound in hydrogen has to be four times that in oxygen. The measured values of the velocities of sound in oxygen and hydrogen are 316ms-1 and 1284ms-1 at NTP. This closely agrees with the predicted values.
According to the kinetic theory of gases, pressure exerted by a gas is given by 
represents the mean square velocity of the molecules of the gas. Substituting in equation (1-28),
Thus, the velocity of sound in gases having the same value of g, varies directly as the root mean square velocity of the molecules of the gas.
Measurement of velocity of sound in a gas enables one to determine the ratio of the principle specific heats (g) and the r.m.s velocity 
of the molecules of the gas.
