120 L of air was mixed with 56 g of nitrogen. determine the amount and mass of nitrogen in the resulting mixture.

It is believed that the average molar mass of the air mixture is 29 g / mol, and it contains 78% by volume of nitrogen. Thus, in 120 liters of nitrogen: 120 * 0.78 = 93.6 liters, or 93.6: 22.4 = 4.18 mol.
Added N2 in an amount of 56 g: 28 g / mol = 2 mol. Total nitrogen became: 4.18 + 2 = 6.18 mol, and the mass is 6.18 * 28 = 173 g.