What is the volume of 21.0 g of nitrogen gas at stp?

We can use the ideal gas law equation to find the volume of N₂ gas at STP
PV = nRT
where 
P - standard pressure - 101 325 Pa
V - volume 
n - number of moles - 21.0 g / 28 g/mol = 0.750 mol 
R - universal gas constant - 8.314 Jmol⁻¹K⁻¹
T - standard temperature - 273 K
substituting the values in the equation 
101 325 Pa x V = 0.750 mol x 8.314 Jmol⁻¹K⁻¹ x 273 K
V = 16.8 L
volume of N₂ at STP is 16.8 L

The air is going to come out because of the low pressure.