Question

The speed of sound in air is 330m/s. What is the mass of 1 mole of air? Assume it is an ideal gas.

(a) 28gm

(b) 38gm

(c) 20.56gm

(d) 29gm

I got this question in a job interview.

My question is based upon Speed of a Travelling Wave topic in portion Waves of Physics – Class 11

Answer 1

Right option is (c) 20.56gm

Explanation: 1 mole of air occupies 22.4L at STP.

Thus, density ρ = m/22.4*10^-3.

Using equation v = √(P/ρ), we get:

v^2 = (P/m)*22.4*10^-3.

Pressure at STP = 10^5Pa.

∴ m = (P/v^2)*22.4*10^-3

= 10^5/330^2* 22.4*10^-3

= 20.56gm.