From ideal gas equation we know that: -

PV = nRT

⇒ PV = m/M x RT

∴

Where, P = pressure of dihydrogen

R = Universal gas constant

T= temperature of dihydrogen

m = mass of dihydrogen

M = molar mass of dihydrogen

V = volume of dihydrogen

∴

Let M be the molar mass of the unknown gas, pressure remains constant. Volume (V) occupied by the unknown gas can be calculated in the same way as above:

∴

According to the question,

∴

= 40gmol^-1

Hence, the molar mass of the gas is 40 g mol^-1.