Limiting Reactant



2H2 + O2 → 2H2O
Each 2 moles of 2H2 need ONE mole of O2 to form 2 moles of H2 O
Assune we have 10 moles of H2, then we can ask the following:
How many moles of O2 are needed?

5 moles

If we have only 10 mol of H2, 4 moles of O2.
Only 8 moles of H2 will be used leaving
2 unreacted(excess) H2 moles.
Meaning that the amount of O2 is limited of H2 needed
→ O2 is called "Limiting Reactant"
→ H2 is called "Excess Reactant"

Hence, at the end of the reaction:

Important Remark:
In a "limiting reactant case", a limiting reactant is used to calculate the number of moles of the product since the limiting reactant is totally consumed in the reaction.

Typically, In a "limiting reactant case", the main task is to find the limiting reactant, then the excess reactant.
Sometimes, finding the excess reactant is easier and will lead to the limiting reactant.


For the reaction: A + B → C + B' (See illustration below):


Exercise on Finding the Limiting Reactant

Check your answers here: Solution to the Exercise on Finding the Limiting Reactant

Important Remarks:

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Date of last modification: Summer , 2019