Question

Nitrogen and hydrogen gases react to form ammonia gas via the following reaction:

Answer 1

1mol N2 + 3mol H2 → 2mol NH3 
Because you are dealing with gases, at the same temperature and pressure you can say: 
1volume N2 + 3 volumes H2 → 2 volumes NH3 
OR: 4 volumes reactants = 2 volumes products. 
You have 1.9+5.7 = 7.6 volumes reactants, which must give 7.6/2 = 3.8 volumes of NH3 
Answer: 3.8 litres of NH3 will be formed