Percentage Yield & Purity (CIE IGCSE Chemistry)

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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:

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%

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:

Answer:

Step 1: Calculate the relative formula mass of the compound

 (2 × 56) + (3 × 16) = 160

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

2 × 56 = 112

Step 3:  Divide the total A_r of the element by the M_r 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:

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 %