Hey there!
From ideal gas equation we know, PV = nRT
All the terms have their usual significance.
PV =w/M * RT
( where , n is number of moles = w/M, we = weight of gas, M = molecular weight of the gas)
Therefore, PV = wRT/M
At standard conditions , value of pressure, P = 1atm and standard temperature, T = 273K
And value of universal gas constant, R = 0.082 lit. atm. K⁻¹.mol⁻¹
Given that, weight of gas,
w = 5.60g and volume of gas,
V = 500mL = 0.5 Liters
Therefore, substituting all the value, we get :
Molar mass = 5.60 * 0.082 * 273 / 1 * 0.5
Molar mass = 250.7232 g/mole
