Limiting and excess reagents

In the chemical equation where we form water from hydrogen and oxygen, we can see that hydrogen and oxygen reacts in a 2:1 ratio. This means that if you have two moles of hydrogen molecules and one mole of oxygen molecules, you have stoichiometric amounts of the reagents, and you form two moles of water.

Formation of two molecules of water presented in an equation and in molecular form. Two molecules of hydrogen that consist of two pairs of two hydrogen atoms plus one molecule of oxygen that consists of two oxygen atoms create two molecules of water, where each water consists of one atom of oxygen and two atoms of hydrogen.

Figure 1: Formation of water molecules.

But what if you only have one mole of hydrogen molecules? Then we say that hydrogen is the limiting reagent, and that we have an excess of oxygen. You can only form as much product as the limiting reagent, so in this case you can only form one mole of water molecules. When calculating the theoretical yield of your reacting, you have to take into account which is the limiting reagent.