A limiting reagent is a reactant which is present in lesser amount in a reaction. When it gets consumed, the reaction will not proceed irrespective of the amount of other reactant present in the reaction. It limits the amount of product formed. In other words, it determines the extent of reaction. Consider the reaction between hydrogen and oxygen to form water. It can be represented as:
2H2 + O2 = 2H2O
From the above reaction, it is clear that 2 moles of hydrogen react with 1 mole of oxygen.
Molar mass of H2 = 2 g
Molar mass of O2 = 32 g
This means that
4 g of H2 reacts with 32 g of O2
3 g of H2 reacts with = (32/4) x 3 g of O2 gas = 24 g
As the given amount of O2 is more than required.
Therefore, O2 is in excess and H2 is limiting reagent here.