Here, we may apply the equation of state for an ideal gas, which is:
PV = nRT
We may substitute the number of moles, n, with:
moles = mass / molecular mass
PV = mRT/Mr
Mr = (mRT)/PV
Substituting the values:
Mr = (81.5 * 0.082 * 307) / (1.75 * 4.92)
Mr = 238.3
The molar mass of the gas is 238.3 grams per mole.