# Limiting reactant

In a chemical reaction, the limiting reactant is the species which is in lowest stoichiometric quantity. In other words, it is the reactant which is consumed before any others.

## Example

Take the chemical reaction

$2H_2 + O_2 \rightarrow 2H_2O$.

Say you have equimolar quantities of hydrogen (H2) and oxygen (O2). Since 2 moles of hydrogen are required for every 1 mole of oxygen, when all the hydrogen is used up there is still leftover oxygen. Thus, hydrogen is the limiting reactant, because it is used up first.