O LevelChemistryCambridgeRevision Notes3. Stoichiometry3.3 The Mole & the Avogadro ConstantPercentage Yield & Purity

Percentage Yield & Purity (Cambridge O Level Chemistry)

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Calculating Percentage Yield, Percentage by Mass & Percentage Purity

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:

percentage space yield space equals space fraction numerator actual space yield over denominator theoretical space yield end fraction space cross times space 100

Worked example

Copper(II) sulfate may be prepared by the reaction of dilute sulfuric acid on copper(II) oxide.

A student prepared 1.6 g of dry copper(II) sulfate crystals.

Calculate the percentage yield if the theoretical yield is 2.0 g.

Answer

  • Actual yield of copper(II) sulfate = 1.6 g
  • Percentage yield of copper(II) sulfate = (1.6 / 2.0) x 100
  • Percentage yield = 80%

Examiner Tip

Percentage yield cannot be greater than 100%, if you calculate a percentage higher than this, you have made an error! The most common error is to divide the theoretical yield by the actual yield so you may just need to swap the number around in your calculation.

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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:

percent sign space of space an space element space equals space fraction numerator total space mass space of space the space element space in space the space compound over denominator relative space formula space mass space of space the space compound end fraction space cross times space 100

Worked example

Calculate the percentage by mass of iron in iron(III) oxide, Fe2O3.

Relative atomic masses, Ar: Fe = 56    O = 16

Answer

Step 1: Calculate the relative formula mass of the compound

 (2 × 56) + (3 × 16) = 160

Step 2: Work out the total Ar of the element you have been asked about in the question, in this case, iron

2 × 56 = 112

Step 3:  Divide the total Ar of the element by the Mr of the compound, then multiply by 100.

(112 ÷ 160) × 100 = 70%

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:

percent sign space purity equals space fraction numerator mass space of space pure space substance over denominator total space mass space of space substance end fraction space straight x space 100

Worked example

A sample of lead(II) bromide was made. It weighed 15 g.

The sample was found to be impure and only contained 13.5 g of lead(II) bromide. 

Calculate the percentage purity of the lead(II) bromide.

Answer

  • The total mass of the substance is 15 g
  • The mass of the pure substance is 13.5 g
  • To calculate the percentage purity:  (13.5 / 15 ) x 100
  • Percentage purity= 90 % 

Examiner Tip

All of these calculations are to find a percentage so don't forget to multiply by 100 to convert your answer to a percentage.

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    Caroline

    Author: Caroline

    Expertise: Physics Lead

    Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.