Percent Yield Definition and Formula

Percent Yield and How to Calculate It

Percent yield is a measure of the efficiency of a chemical reaction.
Percent yield is a measure of the efficiency of a chemical reaction. Adrianna Williams/Getty Images

Percent Yield Definition

Percent yield is the percent ratio of actual yield to the theoretical yield. It is calculated to be the experimental yield divided by theoretical yield multiplied by 100%. If the actual and theoretical yield ​are the same, the percent yield is 100%. Usually, percent yield is lower than 100% because the actual yield is often less than the theoretical value. Reasons for this can include incomplete or competing reactions and loss of sample during recovery. It's possible for percent yield to be over 100%, which means more sample was recovered from a reaction than predicted. This can happen when other reactions were occurring that also formed the product. It can also be a source of error if the excess is due to incomplete removal of water or other impurities from the sample. Percent yield is always a positive value.

Also Known As: percentage yield

Percent Yield Formula

The equation for percent yield is:

percent yield = (actual yield/theoretical yield) x 100%

Where:

  • actual yield is the amount of product obtained from a chemical reaction
  • theoretical yield is the amount of product obtained from the stoichiometric or balanced equation, using the limiting reactant to determine product

Units for both actual and theoretical yield need to be the same (moles or grams).

Example Percent Yield Calculation

For example, the decomposition of magnesium carbonate forms 15 grams of magnesium oxide in an experiment. The theoretical yield is known to be 19 grams. What is the percent yield of magnesium oxide?

MgCO3 → MgO + CO2

The calculation is simple if you know the actual and theoretical yields. All you need to do is plug the values into the formula:

percent yield = actual yield / theoretical yield x 100%

percent yield = 15 g / 19 g x 100%

percent yield = 79%

Usually you have to calculate the theoretical yield based on the balanced equation. In this equation, the reactant and the product have a 1:1 mole ratio, so if you know the amount of reactant, you know the theoretical yield is the same value in moles (not grams!). You take the number of grams of reactant you have, convert it to moles, and then use this number of moles to find out how many grams of product to expect.