Procedures- The first 11 steps: NH,CI and NH 0H are added to the solution. Ni* a

Question

Procedures- The first 11 steps: NH,CI and NH 0H are added to the solution. Ni* and Co will each form complex ions with 4 amine (NHs) groups and Fe and Cr will precipitate as hydroxides. Write the balanced equations for these reactions and indicate any precipitates with (s). 2+ ii. Fe NHsH2O ii. Cr+NH H20 Thioacetamide is used as a source of sulfur ions. After the thioacetamide is added, the Ni complex ion will form a sulfide precipitate. Manganese will also form a sulfide precipitate. Write the balanced equations for these reactions and indicate any precipitates with (s) 2- i. S + 2- What are the possible solid compounds in the Unknown Precipitate 1? Addition of HCl to Precipitate 1 will re-dissolve and separate Fe3 Mn2 and Cr3* ions from NIS. NiS is only slightly soluble in HCI whereas the other 3 ions dissolve readily. What is the possible precipitate in Unknown Precipitate 2? Nis is re-dissolved by adding HCI, HNO, and heat and then solid sulfur is removed to g The confirmation test for Ni? is done by adding NH,OH dropwise until the solution is basic and add dimethylglyomixe. Omit Solution B step 2, the confirmation step for cobalt. Iron and manganese ions can be separated from chromium ions because chromium is an amphote hydroxide. When NaOH and H,02 are added all three ions will initially form hydroxides. As more base is added, the chromium will re-dissolve and form a complex ion and the chromium ions will return to th supernatant. What are the possible precipitates in Unknown Precipitate 3? What are the possible ions in Unknown Solution C? Water is added to Precipitate 3 and the suspended precipitate in divided into 2 tubes to test for Fe and Mn2 respectively. Because we are not testing for Al" or Zn2 you should skip the Solution Cand Precipitate 4 section. Rename Solution C as Solution D and follow the steps for Solution D and Precipitate 5 to confirm the presence or absence of Cr. Do not do the Final Test.

Explanation / Answer

i) Ni2+ + 4 NH3 ---------> [Ni(NH3)4]2+

ii) Fe3+ + 3 NH3 + 3 H2O -------------> Fe(OH)3 (s) + 3 NH4OH

iii) Cr3+ + 3 NH3 + 3 H2O -------------> Cr(OH)3 (s) + 3 NH4OH

i) S2- + [Ni(NH3)4]2+ ---------> NiS (s) + 4 NH3

ii) S2- + Mn ---------> MnS

I do not have any prior information about your sample. However, based on the above reactions, the Precipitate 1 is likely to contain one or more of Fe(OH)3, Cr(OH)3, NiS and MnS.

All the other ions in Precipitate 1 except NiS dissolved in dilute HCl. Hence, Precipitate 2 most likely contains only NiS.

The filtrate from the last step contains Fe3+, Cr3+ and Mn2+. On addition of NaOH and H2O2, all the three ions formed precipitates. However, in excess NaOH, the precipitate due to Cr dissolves leaving behind solid Fe(OH)3 and Mn (most likely as dioxide, MnO2). Therefore, the possible precipitates in Precipitate 3 are Fe(OH)3 and MnO2.

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