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

N2 (g) + 3H2 (g) →2NH3 (g)

At a certain temperature and pressure, 1.9 L of N2 reacts with 5.7 L of H2.

If all the N2 and H2 are consumed, what volume of NH3, at the same temperature and pressure, will be produced?

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