Tests for anions and ammonium ionsEdexcel International A Level Chemistry: Subtopic test

1. 1
   A student adds dilute hydrochloric acid to a white powder and sees vigorous effervescence. She passes the gas given off through limewater, which turns milky. The powder is known to be a sodium salt.

   (a)

   Which ions could be present in the powder?

   [1 mark]

   • ASulfate ions
   • BAmmonium ions
   • CCarbonate or hydrogencarbonate ions
   • DChloride ions

   (b)

   What is the white solid that makes the limewater milky?

   [1 mark]

   • ACa(OH)₂
   • BCaCO₃
   • CCaO
   • DCaSO₄

   (c)

   The powder is sodium carbonate. A 0.53 g sample reacts with excess dilute hydrochloric acid. Calculate the maximum volume of carbon dioxide produced, in cm³, at room temperature and pressure. (Na₂CO₃: M = 106.0 g mol⁻¹; molar volume of a gas at room temperature and pressure = 24.0 dm³ mol⁻¹)

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A technician at a swimming pool tests a sample of pool water for dissolved sulfate. He adds dilute hydrochloric acid and then barium chloride solution, and a white precipitate forms.

   (a)

   Why is dilute hydrochloric acid added before the barium chloride solution?

   [1 mark]

   • ATo make the barium chloride dissolve
   • BTo react with the sulfate ions
   • CTo precipitate any chloride ions
   • DTo remove carbonate ions, which would also give a white precipitate with barium ions

   (b)

   Which is the ionic equation for the reaction that forms the precipitate?

   [1 mark]

   • ABa²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
   • BBa²⁺(aq) + 2Cl⁻(aq) → BaCl₂(s)
   • CH⁺(aq) + SO₄²⁻(aq) → HSO₄⁻(aq)
   • DBa²⁺(aq) + CO₃²⁻(aq) → BaCO₃(s)

   (c)

   Explain why dilute sulfuric acid should not be used instead of dilute hydrochloric acid to acidify the sample.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A soil-testing laboratory analyses a solid fertiliser that is thought to be ammonium sulfate, (NH₄)₂SO₄. Relative atomic masses: H 1.0, N 14.0, O 16.0, S 32.1, Ba 137.3.

   (a)

   Describe how the laboratory could show that the fertiliser contains ammonium ions. Give the ionic equation for the reaction and the observation that identifies the gas.

   [3 marks]

   (b)

   A 1.32 g sample of pure ammonium sulfate is dissolved in water and excess acidified barium chloride solution is added. Calculate the mass of precipitate formed. Give your answer to three significant figures.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A forensic laboratory has the following reagents: dilute hydrochloric acid, aqueous barium chloride, aqueous sodium hydroxide, limewater, and damp red and blue litmus paper. The laboratory uses them to test aqueous samples for carbonate, sulfate and ammonium ions.

   (a)

   Three colourless aqueous solutions, P, Q and R, are ammonium carbonate, sodium sulfate and ammonium sulfate in an unknown order. Describe a series of tests that would identify each solution, giving the observations and the ionic equations.

   [6 marks]

   (b)

   A student has a solution containing both carbonate ions and sulfate ions. She adds barium chloride solution and sees a white precipitate, and concludes that sulfate ions are present. Evaluate her conclusion and describe how the presence of both ions could be shown correctly.

   [6 marks]

   Total for question 4: 12 marks

End of questions