At stp ( standard temperature and pressure) normally a temperature of 273 kelvin and a temperature of 1 atmosphere, 1 mole of a gas occupies a volume of 22400 ml or 22.4 liters.
Therefore, 33.6 liters of nitrogen contain; 33.6 ÷ 22.4 = 1.5 moles
But, 1 mole of nitrogen gas = 28 g
Thus 1.5 moles will have a mass of 1.5 × 28 = 42g
so, 33.6 l of nitrogen gas contain a mass of 42 gramms
