Percentage Yield & Purity

Calculating Percentage Yieldvideo

Calculating Percentage Yield

Percentage Yield

• Yield is the term used to describe the amount of product you get from a reaction

• In practice, you never get 100% yield in a chemical process for several reasons

• These include:

  • Some reactants may be left behind in the equipment

  • The reaction may be reversible and in these reactions a high yield is never possible as the products are continually turning back into the reactants

  • Some products may also be lost during separation and purification stages such as filtration or distillation

  • There may be side reactions occurring where a substance reacts with a gas in the air or an impurity in one of the reactants

  • Products can also be lost during transfer from one container to another

Actual & Theoretical Yield

• The actual yield is the recorded amount of product obtained

• The theoretical yield is the amount of product that would be obtained under perfect practical and chemical conditions

• It is calculated from the balanced equation and the reacting masses

• The percentage yield compares the actual yield to the theoretical yield

• For economic reasons, the objective of every chemical producing company is to have as high a percentage yield as possible to increase profits and reduce costs and waste

• To calculate percentage yield the following equation is used:

$\mathrm{percentage} \mathrm{yield} = \frac{\mathrm{actual} \mathrm{yield}}{\mathrm{theoretical} \mathrm{yield}} \times 100$

Calculating Percentage Mass and Percentage Uncertaintyvideo

Calculating Percentage Mass and Percentage Uncertainty

Calculating percentage mass 

• You may be asked to find the percentage by mass of an element within a compound

• This can be found by calculating the relative formula mass of the compound and then using the following equation:

$\% \mathrm{of} \mathrm{an} \mathrm{element} = \frac{\mathrm{total} \mathrm{mass} \mathrm{of} \mathrm{the} \mathrm{element} \mathrm{in} \mathrm{the} \mathrm{compound}}{\mathrm{relative} \mathrm{formula} \mathrm{mass} \mathrm{of} \mathrm{the} \mathrm{compound}} \times 100$

Percentage Purity

• A pure substane has nothing else mixed with it

• Often, the product you are trying to obtain may become contaminated with unwanted substances such as unreacted reactants, catalysts and other impurities

• To calculate the percentage purity the following equation is used:

$\% \mathrm{purity}= \frac{\mathrm{mass} \mathrm{of} \mathrm{pure} \mathrm{substance}}{\mathrm{total} \mathrm{mass} \mathrm{of} \mathrm{substance}} \mathrm{x} 100$