Limiting reagent: The substance that is totally consumed when the chemical reaction is complete. The amount of product formed is limited by this reagent, since the reaction cannot continue without it.

Excess reactant: The reactant in a chemical reaction with greater amount than necessary to react completely with the limiting reagent.

Example 1: Consider the combustion of methane in the laboratory. The limiting reagent is methane, the excess reactant is oxygen - the combustion is not going to use up all the supply of oxygen in the room.

Example 2: Consider the equation \[4Al +3O_2 \rightarrow 2Al_2O_3\]

Formula weights:

Aluminium: 27

Oxygen: 32

Imagine you have 64g of oxygen and 54g of aluminium. Which is the limiting reagent?

64g of oxygen is 2 moles. 2 moles of oxygen would require 8/3 moles or 72g of aluminium to fully react.

54g of aluminium is also 2 moles. 2 moles of aluminium would require 3/2 moles or 24g of oxygen to react.

Therefore, there is enough oxygen for all of the aluminium to be reacted with, but not enough aluminium for all of the oxygen to be fully reacted with. Therefore, the limiting reagent must be the aluminium.