Question

(Will give medal!)
Air is about 78% nitrogen gas (N2) by mass. The molar mass of nitrogen gas is 28.0 g/mol. A 100.0-g sample of air contains how many moles of nitrogen?

2.8
3.6
28
78

Answer 1

78 % by mass mean.
100 g of air contain 78 g N2.
moles of Nitrogen = mass/m.mass=78/28=2.8 moles