Question

air is about 78% nitrogen gas, the molar mass of nitrogen gas is 28.0g/mol. A 100.0 sample of air contains how many moles of nitrogen

Answer 1

when you said that 78% of the air is nitrogen that's means that in 100 g of air you will have 78g of nitrogen.

78 g N x 1 mol of N /28g = moles of nitrogen in 100 g of air