Final answer:
The excess reactant left after a reaction depends on the reaction conditions and can be calculated by subtracting the amount of excess reactant consumed from the total amount given.
Step-by-step explanation:
In a chemical reaction, the reactant that is completely used up and determines the amount of product formed is called the limiting reactant. The reactant that remains after the reaction is completed is called the excess reactant. The amount of excess reactant left over depends on the reaction conditions and can be calculated by subtracting the amount of excess reactant consumed from the total amount given.