Question

A student analyzing an unknown solution containing some of the cations studied in this module records the following results. At each step, indicate which cations are present and which have been proven absent from the unknown mixture.1) 4 drops of 6M HCl solution are added to the unknown solution and no precipitate2) The pH of the solution from (1) is raised to 9.5 by the addition of 6M NH3 solution. A cloudy, red-brown reaction mixture results3) The solution from (2) is centrifuged, and the colorless supernatant liquid is decanted into a separate container for later analysis. The precipitate is rinsed, and 6M NaOH solution is added. The mixture is heated and then centrifuged, resulting in a red-brown precipitate and a clear, colorless supernatant liquid.4) The precipitate from (3) is dissolved in 6M HCl solution, the pH is adjusted to 5, and 0.1M K4Fe(CN)6 solution is added to the precipitate, resulting in the formation of a dark blue precipitate in a blue solution5) When 6M HCl, aluminon, and 6M NH3 solutions are added to the second supernatant liquid from (3), a clear, red solution results.6) When 0.5M (NH4)2C2O4 solution is added to the first supernatant liquid from(2), a clear, colorless solution results7) The solution from (6) is heated to dryness with HNO3, and the residue dissolved in 6M HCl solution. The solution remains clear and colorless following the adjustment of the pH of the solution to 12 with the addition of 6M and 1M NaOH solution8) When 6M CH3COOH followed by 0.1M Na3Co(NO2)6 solutions are added to the solution from (7), a yellow precipitate forms.