When nitrogen gas reacts with hydrogen gas, ammonia gas is formed. how many grams of hydrogen gas are required to react completely with 3.5 l of nitrogen at stp?

The balanced equation for the formation of ammonia is as follows
N₂ + 3H₂ ---> 2NH₃
stoichiometry of N₂ to H₂ is 1:3
we need to find the moles of N₂, volume of N₂ has been given 
molar volume is where 1 mol of any gas occupies a volume of 22.4 L at STP.
if 22.4 L is occupied by 1 mol 
then 3.5 L of gas is occupied by - 3.5 L / 22.4 L/mol = 0.16 mol 
number of moles of N₂ present - 0.16 mol
1 mol of N₂ requires 3 mol of H₂
therefore 0.16 mol of N₂ requires - 3 x 0.16 = 0.48 mol of H₂
mass of H₂ required - 0.48 mol x 2 g/mol = 0.96 g
0.96 g of H₂ is required

Answer is to avoid experimental error

True, the standard unit for volume is liters which can be represented by either a lower case "l" or a capital "l"