Are you referring to this reaction?
N2 + 3H2 --> 2NH3
If so, then let us work it out.
6 liters of H2 at STP is: We will be using stochiometry on the equation, therefore, in ever 3mol of H2 that you react, you have to react 1 mol of N2 too.
6L (1000mL/L) ( cm^3/mL) (m/100cm)^3 ( .0899kg/m^3 )(1000g/kg) (1mol/ 2.016g)
= .2678mols of H2.
So,
.2678mols of H2 ( 1mol N2 / 3mols of H2) is .08927 mols N2
Then, by using the properties of nitrogen, continue working it out by doing the same conversion backwards:
.08927mols x 28.02 g/mol x kg/1000g x 1m^3 / 1.2506kg x (100cm/m)^3 x mL/cm^3 x L/1000mL = 2L
Therefore, 2 Liters is required for the reaction above.