Percentage Yield and Purity

A series of free IGCSE Chemistry Activities and Experiments (Cambridge IGCSE Chemistry).

Calculating Percentage Yield
How to calculate the percentage yield for a reaction?
Explain why percentage yield may be less than 100%
Example:
Calculate the mass of magnesium sulfate that could be produced from 48 g of magnesium. Assume that the sulfuric acid is unlimited.
It is not always possible to achieve 100% yield in a chemical reaction.
• Some of the product may be lost when it is separated from the reaction mixture.
• Some of the reactants may react in different ways to the expected reaction so we do not get the product we expect.
• Reversible reactions may not go to completion.
Example:
A scientist reacted 48 g of magnesium and produced 150 g of magnesium sulfate. Calculate the percentage yield.
Percentage purity and yield
Example:
When 24.0 g of an impure sample of aluminium was reacted with iron oxide, 45.7 g of iron was formed. Calculate the percentage purity of the sample of impure aluminium.
Step 1: Calculate the number of moles of Fe - this is the pure, known chemical and the one to convert into moles.
Step 2: Use the equation to work out how many moles of Al would have been needed and convert this to a mass.
Step 3: Percentage purity = mass pure/mass impure x 100
Example:
Using the previous equation, 1.35 g of aluminium reacts to form 2.10 g of aluminium oxide. What percentage of aluminium was converted into its oxide?
Step 1: Calculate the number of moles of Al - this is the known chemical.
Step 2: Use the equation to work out how many moles of Al2O3 would have been made and convert this to a mass.
Step 3: % yield = Mass product made/maximum mass product x 100

Questions:
1. Consider the reaction of C6H6 + Br2 → C6H5Br + HBr
a. What is the theoretical yield of C6H5Br if 42.1 g of C6H6 react with 73.0 g of Br2?
b. If the actual yield of C6H5Br is 63.6 g, what is the percent yield?

2. Use the following reaction:
C4H9OH + NaBr + H2SO4 → C4H9Br + NaHSO4 + H2O
If 15.0 g of C4H9OH react with 22.4 g of NaBr and 32.7 g of H2SO4 to yield 17.1 g of C4H9Br, what is the percent yield of this reaction?

3. Silicon nitride (Si3N4) is made by a combining Si and nitrogen gas (N2) at a high temperature.
How much (in g) Si is needed to react with an excess of nitrogen gas to prepare 125 g of silicon nitride if the percent yield of the reaction is 95.0%?

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