When we combine two or more reactants there is usually too much of one and not enough of the other. Obviously the reaction will stop when one reactant is completely used up.

Three scenarios exist a reactant can be in:

excess, where there is obviously too much of the reactant present and not all will be used up;

limiting, where the reactant will be used up completely before the other reactant is used up;

The right ratio (proportion) with all other reactants and all will be used up in the reaction.

So the amount of product formed depends on the limiting reactant. When the limiting reactant runs out the reaction will stop. It is similar to a recipe where the cooking stops when an essential ingredient runs out.

So how do we deal with excess and limiting reagents?. We can best explain this with an example.

Aluminium reacts with oxygen according to the reaction below.

4Al + 3O₂ => 2Al₂O₃

If 54 grams of aluminium reacts with 32 grams of oxygen what mass of aluminium oxide is formed?

Step 1 Convert to moles all the reactants
Moles of aluminium = 54 grams/27 atomic mass units = 2 moles
Moles of Oxygen gas = 32 grams/ 32 atomic mass units = 1mole

Step 2 Identify the reactant that is the LIMITING reactant. (In other words the reactant that will stop the reaction and the formation of product)
Take aluminium and ask the following questions.
How much do we have? = 2moles
How much oxygen do we need to react with this amount of aluminium? = 3/4 X 2 moles = 1.5 mole of oxygen.
Do we have enough ? NO! oxygen is therefore our limiting reactant.

We now use the oxygen to work out the mass of aluminium oxide produced..

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