Percentage Yield

When you carry out a chemical reaction in the lab, it’s important to know how much product you’ll get out of it. The amount of product you actually get is called the actual yield, while the theoretical yield is the maximum amount of product you could get if everything goes perfectly. Let’s take a closer look at both of these concepts.

Theoretical Yield

The theoretical yield is the amount of product that would be produced if the reaction proceeded under ideal conditions. In other words, it’s the maximum amount of product that could be produced from a given amount of reactants. However, it’s rare to achieve this yield in practice because there are many factors that can affect the reaction.

Some reasons why the theoretical yield is difficult to obtain include:

  • Loss of Reactants – Some reactants can be lost during separation, such as filtration.
  • Incomplete Reaction – Not all of the reactants may react, leaving some of the mixture as reactants. For example, this could happen if it is a slow reaction or a reversible reaction, so products keep turning back into reactants.
  • Side Reactions – Side reactions can occur when a gas reacts with impurities or gases in the air, producing reactants that are different from the expected product.

Actual Yield

The actual yield is the amount of product you actually obtain from the reaction in the lab. The actual yield is always less than or equal to the theoretical yield because it’s rare to achieve ideal conditions in the lab.

However, if everything in the lab goes perfectly (a rare occurrence), the two yields can be the same.

Calculating Percentage Yield

We can calculate the percentage yield using this equation:

The percentage yield is a measure of the efficiency of a chemical reaction. It tells us how much of the expected product we actually obtained. The formula for calculating percentage yield is:

Example

In an experiment, aluminium oxide reacts with sulfuric acid to form aluminium sulfate and water. The balanced equation for the reaction is:

Aluminium oxide + Sulphuric acidAluminium sulfate + Water

Al2O3 + 3H2SO4 Al2(SO4)3 + 3H2O

The reaction formed 2.4 g of dry aluminium sulfate crystals. If the theoretical yield is 3.0 g, calculate the percentage yield of aluminium sulfate.

It is given that the theoretical yield of the reaction is 3.0 g, which means that in perfect conditions, we would expect to obtain 3.0 g of aluminium sulfate. However, the actual yield obtained in the experiment was 2.4 g of dry aluminium sulfate crystals.

Using the formula for percentage yield, we can calculate the efficiency of the reaction:

percentage yield = (actual yield / theoretical yield) × 100

= (2.4 g / 3.0 g) × 100

= 0.8 × 100

Percentage yield = 80%

Therefore, the percentage yield of the reaction is 80%, indicating that only 80% of the theoretical yield was obtained in the experiment.

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