Stoichiometry - Limiting and Excess Reactant

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The following Stoichiometry Road Map gives a summary of how to use stoichiometry to calculate moles, masses, volumes and particles in a chemical reaction with limiting and excess reactants. Scroll down the page for more examples and solutions.

 

Stoichiometry - Limiting and Excess Reactant
Introduction to Limiting Reactant and Excess Reactant
The limiting reactant or limiting reagent is the first reactant to get used up in a chemical reaction.
Once the limiting reactant gets used up, the reaction has to stop and cannot continue and there is extra of the other reactants left over. Those are called the excess reactants.
We will learn about limiting reactant and limiting reagent by comparing chemical reactions to cooking recipes and we will look at an actual stoichiometry problem.
Example:
What is the greatest amount of NH₃ (in moles) that can be made with 3.2 moles of N₂ and 5,4 moles of H₂? What is the limiting reactant? Which reactant is in excess and how many moles of it are left over?

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Limiting Reactant Practice Problem (moles)
To solve stoichiometry problems with limiting reactant or limiting reagent:

1. Figure out which of the reactants is the limiting reactant or limiting reagent.
2. See how much product can be formed by using the maximum amount of the limiting reactant or limiting reagent.
3. The excess reactant is what is left over after all of the limiting reactant has been used up.

Example:

1. What is the greatest amount of MgO (in moles) that can be made with 7.8 moles of Mg and 4.7 moles of O₂? Which is the limiting reactant? Which reactant is in excess and how many moles of it are left over?

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Limiting Reactant Problem (grams)

Example:

1. What is the greatest amount of AlCl₃ (in grams) that can be made with 114 grams of Al and 186 grams of Cl₂? Which is the limiting reactant? Which reactant is in excess, and how many grams of it are left over?

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Questions:

1. Take the reaction:
   NH₃ + O₂ → NO + H₂O.
   In an experiment, 3.25 g of NH₃ are allowed to react with 3.50 g of O₂.
   a. Which reactant is the limiting reagent?
   b. How many grams of NO are formed?
   c. How much of the excess reactant remains after the reaction?

2. If 4.95 g of ethylene (C₂H₄) are combusted with 3.25 g of oxygen.
   a. What is the limiting reagent?
   b. How many grams of CO₂ are formed?

3. A reaction container holds 5.77 g of P₄ and 5.77 g of O₂.
   The following reaction occurs: P₄ + O₂ → P₄O₆.
   If enough oxygen is available then the P4O6 reacts further: P₄O₆ + O₂ → P₄O₁₀.
   a. What is the limiting reagent for the formation of P₄O₁₀?
   b. What mass of P₄O₁₀ is produced?
   c. What mass of excess reactant is left in the reaction container?

• Show Solutions

  1. a. O₂
     b. 2.63 g NO
     c. 1.76 g NH₃ left

  2. a. O₂
     b. 2.98 g CO₂

  3. a. O₂
     b. 5.78 g P₄O₁₀
     c. 5.76 g P₄O₁₀ remain

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