Question

The density of a gas at S.T.P. is 1.40g/l. The molecular weight of the gas is:

Answer 1

At S.T.P., 1 mol of a gas has a volume of 22.4 litres
mol. wt. of a gas = $density\left(g/l\right)\times 22.4l/mol$

$1.4g/l\times 22.4l/mol=31.36\approx 31.4g/mol$