22.4 litres of a gas weighs 70 g at S.T.P. Calculate the weight of the gas if it occupies a volume of 20 litres at ${27}^{\circ }C$ and 700 mm Hg of pressure.

Use the Ideal gas law to find the number of moles of the gas
PV=nRT
Where gas constant, R = 0.0821 atm L/mol K
Pressure, P = 700/760 = 0.921 atm
Temperature, T = 27°C +273 = 300 K
n = PV / RT

= (0.921atm x 20 atm) / (0.0821 atm L/mol K x 300 K)
n = 0.748 moles

70 grams of the 22.4 L of gas has 1 mol at STP.

It means 1 mol has 70 gram, therefore it is 70 g/mol (molar mass)

Mass of the gas = Moles x Molar mass

= 0.748 moles x 70 g/mol = 52.414 grams.