What is the mass of 4.5 moles of nitrogen gas

n = m / M

m = n × M

Nitrogen (N2) - a gas is always with an index 2 if we don't have shown how many molecules are there

m (N2) = 4,5 mol × (2×14.007 g/mol)

m (N2) = 4,5 mol × 28.014 g/mol = 126.063g

if you don't solve with the decimals in class just replace 14.007 with 14

also if you haven't learned abour gasses aways having index 2 then:

m (N2) = 4,5 mol × 14.007 g/mol = 63.0315g