Answer to Question #164272 in General Chemistry for emma

Question

You are working as a laboratory technician in a water quality testing laboratory. You have recently received some water samples believed to contain lead (Pb⁺²), barium (Ba⁺²), and magnesium (Mg⁺²) ions. Your laboratory has been asked to identify and separate the ions out of the water samples using precipitation reactions, and verify the identity of the metal ions using flame tests.

Your task is to design an experimental procedure to precipitate out each of the three metal ions from the water sample, separate the precipitate from the liquid supernatant after each precipitation, and then verify the identify of the metal ions in the precipitates using flame tests.

To conduct your precipitations you have aqueous solutions of sodium chloride, NaCl_(aq), sodium sulfide, Na₂S_(aq), and sodium hydroxide, NaOH_(aq).

Answer 1

Ba⁺² does not give a hydroxide precipitate because barium hydroxide, Ba(OH)₂, is too soluble.

lead(II) ion: Pb²⁺_(aq) + 2OH^–_(aq) ==> Pb(OH)_2(s

a white precipitate forms

of lead(II) hydroxide, which dissolves in excess sodium hydroxide (amphoteric) to give a clear colourless solution but does not dissolve in excess ammonia solution.

magnesium ion: Mg²⁺_(aq) + 2OH^–_(aq) ==> Mg(OH)_2(s) gives a white precipitate

of magnesium hydroxide with ammonia or sodium hydroxide, which is not soluble in excess of either NH₃ or NaOH. You could distinguish Mg from Ca with a <b>flame test</b> or ammonia test above.