Exercises: Identification of Anions

Dilute hydrochloric acid is added to solid sodium carbonate and effervescence is observed.

1. a) Name the gas given off.
2. b) State the test used to identify the gas.
3. c) State the result of that test.

Silver nitrate solution, $\mathrm{AgNO_3(aq)}$, is added to a solution of sodium chloride acidified with dilute nitric acid.

1. a) State what is observed.
2. b) Name the precipitate formed.
3. c) Write the ionic equation for the reaction.

Barium chloride solution is added to a solution of sodium sulfate, and a white precipitate forms.

1. a) Name the precipitate.
2. b) Write the ionic equation for its formation.

A student adds dilute hydrochloric acid to a solid and observes a gas that turns limewater milky.

1. a) Name two possible anions in the solid.
2. b) Suggest a further test that distinguishes between them, and state the expected results for each anion.
3. c) Write the equation for the reaction of hydrogencarbonate ions with acid.

Dilute acid is added to a solid and a colourless gas with a sharp, choking smell is produced. The gas turns acidified potassium dichromate(VI) paper from orange to green.

1. a) Identify the gas and the anion present.
2. b) Write an ionic equation for the reaction between the anion and the acid.
3. c) Explain why the colour change of the dichromate(VI) shows the gas is a reducing agent.

A solution of an unknown salt gives a yellow precipitate with silver nitrate solution after adding dilute nitric acid. The precipitate is soluble in ammonia only in concentrated form, if at all, while a white silver chloride precipitate dissolves easily in dilute ammonia.

1. a) Suggest why the solution is first acidified with nitric acid before adding silver nitrate.
2. b) Identify the type of anion that could give a yellow precipitate, and one possible anion in the phosphate family.
3. c) Write an ionic equation for the formation of silver phosphate, $\mathrm{Ag_3PO_4}$.

Four unlabelled bottles each contain a different white sodium salt: sodium chloride, sodium sulfate, sodium carbonate and sodium nitrate. Design a sequence of tests using only dilute nitric acid, silver nitrate solution and barium nitrate solution to identify each salt. For each test state the observation expected and write ionic equations for all precipitation reactions.

A student adds barium chloride solution to a solution of an unknown sodium salt and gets a white precipitate. Dilute hydrochloric acid is added and the precipitate dissolves with effervescence, producing a gas that turns limewater milky.

1. a) Identify the anion present and the precipitate.
2. b) Explain why the precipitate would not dissolve if the anion were sulfate.
3. c) Explain why hydrochloric acid, rather than sulfuric acid, is used in this test.
4. d) Write balanced ionic equations for the precipitation and for the dissolving of the precipitate.

A $25.0\ \text{mL}$ sample of a solution contains sulfate ions. Excess barium chloride solution is added and the precipitate is filtered, dried and weighed. The mass of barium sulfate is $0.233\ \text{g}$. $(A_r$: Ba = 137.3, S = 32.1, O = 16.0$)$

1. a) Calculate the moles of barium sulfate formed.
2. b) Calculate the concentration of sulfate ions in the original sample, in $\text{mol L}^{-1}$.
3. c) Explain why the sample is acidified before adding barium chloride.
4. d) State one source of error that would make the calculated concentration too high.