Limiting reactant calculations

Often in many chemical processes a reactant is present in excess. The reactant that is present in excess is the chemical that remains after the reaction is completed. The reactant that is fully consumed during the reaction is called the limiting reagent. In combustion reactions, oxygen gas is usually present in excess in order for the the reaction to under complete combustion. Complete combustion means that the products of the reaction are CO₂ (g) and H₂O (l). Contrast to complete combustion is incomplete combustion. Incomplete combustion tends to form CO (g) as well as CO₂ (g).

Example

(a) How many grams of carbon dioxide gas is produced when 20.5 grams of ethyl alcohol, C₂H₅OH, burns with 100.0 grams of oxygen, O₂?

(b) Identify the limiting reagent and how much of the other reactant is left over?

Step 1: Identify the chemical equation involved: a combustion reaction.

Step 2: Write the balanced chemical equation.

Step 2a: Convert the mass of C₂H₅OH to moles of C₂H₅OH.

Step 2b: Convert the mass of O₂ to moles of O₂.

Step 3: Determine the stoichiometric ratio of the substances that you are working with, specifically:

• between C₂H₅OH and O₂
• between C₂H₅OH and CO₂
• between O₂ and CO₂

Step 4: Using the moles of C₂H₅OH and O₂ calculated in Step 2a and 2b, compare the mole ratio of C₂H₅OH : O₂ to that obtained in Step 3. Determine which is present as the limiting reagent and which is present in excess.

C₂H₅OH is the limiting reactant.

Step 5: Calculate the amount of reactant that is left over.

External link

Limiting reagent

Content suitability

BCIT courses: CHEM 0011