If 6 liters of hydrogen gas are used, how many liters of nitrogen gas will be needed for the above reaction at stp?

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.