Core practical - Making ammonium sulfate

Question 1

Method: (8)

Answer 1

1- Put some dilute sulfuric acid into a beaker.
2- Add a few drops of methyl orange indicator.
3- Add dilute ammonia solution drop by drop, stirring in between.
4- Continue step 3 until the colour permanently changes from red to yellow.
5- Add a few more drops of dilute ammonia solution.
6- Pour the reaction mixture into an evaporating basin, and heat carefully over a boiling water bath.
7- Stop heating before all the water has evaporated. Leave aside for crystals to form.
8- Pour away excess water and leave the crystals to dry in a warm oven (or pat dry with filter paper).

Question 2

Explain why a few more drops of dilute ammonia solution are added at step 5, after the end-point. (2)

Answer 2

This is to make sure that all the dilute sulfuric acid has reacted.
During heating at stage 6, any excess ammonia gas evaporates and leaves the evaporating basin, so only ammonium sulfate solution is present.