Chemistry 11
Limiting Reactants
Most chemical reactions continue until one of the reactants is
consumed or used up.
The reactant that is completely used up and therefore, stops the
reaction is called the Limiting Reactant.
All other reactants are said to be in excess.
Steps
1. Write the balanced equation
2. Change the quantities of reactants to moles
3. Use mole ratios to determine the limiting reactant
4. Complete the problem using the quantity of moles of the
limiting reactant
Example 1
If 40.0g of H3PO4 reacts with 60.0g of MgCO3 calculate the
volume of CO2 produced at STP.
Step 1 Write the balanced equation
2H3PO4 + 3 MgCO3  Mg3(PO4)2 + 3 CO2
+
3H2O
Step 2 Change the quantities of reactants to moles
𝑚𝑜𝑙𝐻3 𝑃𝑂4 =
40𝑔
= 0.41 𝑚𝑜𝑙
98.03 𝑔⁄𝑚𝑜𝑙
𝑚𝑜𝑙 𝑀𝑔𝐶𝑂3 =
60𝑔
= 0.71𝑚𝑜𝑙
84.3 𝑔⁄𝑚𝑜𝑙
The molar quantities calculated above tell you how much of each
reactant you HAVE.
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Chemistry 11
Step 3 Use mole ratios to determine the limiting reactant
Choose one of the reactants and set up a molar ratio between the
chosen reactant and the other reactant.
0.41 𝑚𝑜𝑙 𝐻3 𝑃𝑂4 𝑥
3 𝑚𝑜𝑙𝑀𝑔𝐶𝑂3
= 0.612 𝑚𝑜𝑙𝑀𝑔𝐶𝑂3
2 𝑚𝑜𝑙𝐻3 𝑃𝑂4
The above calculation tells you how much of the reactant, MgCO3
you NEED in order to completely react with all of the H3PO4.
According to the above calculation you NEED 0.612 mol of
MgCO3, you HAVE 0.71 mol. Therefore, MgCO3 is not limiting
because you HAVE more than you NEED (this reactant is in
excess). As a result the H3PO4 is the limiting reactant.
Step 4 Complete the problem using the quantity of moles of the
limiting reactant
0.41𝑚𝑜𝑙 𝐻3 𝑃𝑂4 𝑥
3 𝑚𝑜𝑙 𝐶𝑂2
= 0.615 𝑚𝑜𝑙𝐶𝑂2
2 𝑚𝑜𝑙 𝐻3 𝑃𝑂4
0.615 𝑚𝑜𝑙𝐶𝑂2 𝑥
22.4𝐿
= 13.78𝐿 𝐶𝑂2
1 𝑚𝑜𝑙
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Chemistry 11
Calculating the amount of excess reagent.
We know that we have more MgCO3 than we need. Therefore,
there will be some MgCO3 left over after the reaction is complete.
This amount is the excess. To calculate the amount of MgCO3
left over. Calculate the difference between the amount you have
and the amount you need.
HAVE 0.71 mol - NEED 0.612 mol = 0.098 mols in excess.
You can then convert from moles to the requested volume, mass,
or number of particles as requested in the question.
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