A pale green crystalline salt, S, is used as a standard in some chemical analyses. S contains two different cations and one type of anion. A student carries out these tests on a solution of S.
The table shows the results.
| Test | Observation |
|---|---|
| 1. Add sodium hydroxide solution | green precipitate forms; on standing, the surface of the precipitate turns orange-brown |
| 2. Warm the mixture from test 1 | a gas is given off that turns damp red litmus paper blue |
| 3. Add dilute hydrochloric acid, then barium chloride solution | white precipitate |
| 4. Flame test | no characteristic colour |
Identify the three ions present in S. Use the results in the table to explain your answer.
Explain the change in colour of the precipitate in test 1 on standing.
Write the ionic equation for the reaction that produces the gas in test 2.
In S, there are 2 ammonium ions for every 1 cation of the metal. Deduce the formula of S. Explain your answer.
S is a hydrated salt with 6 water molecules in each formula unit.
Calculate the percentage by mass of iron in S. Give your answer to three significant figures. Relative atomic masses: H = 1, N = 14, O = 16, S = 32, Fe = 56
35 exam-style questions on Edexcel GCSE Chemistry 10.1 Qualitative analysis: tests for ions, covering 10.1.1 Principles of ion tests and flame tests, 10.1.2 Tests for cations and for ammonia, 10.1.3 Tests for anions, 10.1.4 Identification of ions in unknown salts, and 10.1.5 Instrumental methods of analysis. Each one has a worked solution and a mark scheme showing where the marks go.