Describe how a pure, dry sample of ammonium sulfate could be prepared in the laboratory starting from ammonia solution and dilute sulfuric acid.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Ammonium sulfate is a soluble salt used as a fertiliser. It is formed by the reaction between ammonia solution and dilute sulfuric acid.
Model answer (4 marks)
1. Add a measured volume of ammonia solution to a measured volume of dilute sulphuric acid in a beaker, stirring.
2. Use a pH indicator (e.g. litmus) to watch the solution change colour; when the colour indicates neutrality (or after the reaction stops), stop adding acid.
3. Heat the neutral solution gently to evaporate excess water, concentrating the solution.
4. Cool the concentrated solution; ammonium sulphate crystals will precipitate. Filter the crystals, wash with cold water, then dry them in a warm oven or between filter papers until constant mass.
2. Use a pH indicator (e.g. litmus) to watch the solution change colour; when the colour indicates neutrality (or after the reaction stops), stop adding acid.
3. Heat the neutral solution gently to evaporate excess water, concentrating the solution.
4. Cool the concentrated solution; ammonium sulphate crystals will precipitate. Filter the crystals, wash with cold water, then dry them in a warm oven or between filter papers until constant mass.
Examiner tips
- Use the correct order of addition (acid first, then base) to avoid excess acid. Show the use of an indicator or a titration step to reach neutrality. Explain that heating concentrates the solution and cooling allows crystal formation. Mention filtration and drying to obtain a pure, dry product.
Common mistakes
- Adding ammonia to ammonia or acid to acid, causing excess acid. Failing to recognise when the solution is neutralised. Not concentrating the solution before cooling, leading to no crystal formation. Leaving water in the final product, giving a wet sample.
Mark scheme (4 marks)
- Add ammonia solution to dilute sulfuric acid (using an indicator or until acid is neutralised / until no further reaction)
- Use an indicator (such as litmus) to determine when the solution is neutral, then repeat without indicator / or add exact volumes determined from titration
- Evaporate (heat) the neutral solution to concentrate it / reduce the volume of water
- Allow the solution to cool so that ammonium sulfate crystals form, then filter off the crystals and dry them (e.g. between filter papers or in a warm oven)
Related
- All Cambridge International IGCSE Chemistry (0620) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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