Question

For the reaction below, how many liters of ammonia form when 7.5liters of hydrogen react?

Answer 1

As 1 mole of all gases occupy the same volume at the same temperature and pressure we can use moles OR volumes for gas reactions at a given temperature and pressure. 
From the equation 3 moles (and 3 volumes) of H2 will produce 2 moles (and 2 volumes) of NH3. 

So the volume of NH3 produced is 2/3 times the volume of H2 consumed = 3/2 x 7.5 = 5 litres.