Question

Below is an analysis scheme for a mixture of Co2+, Cu²+, Fe3+, and Ni2+. You are asked for the formula of each cited precipitate and for the formula of the cation in the final solution. Include the net ionic chemical equations for the precipitation reactions. Step 1. In test tube 11, potassium nitrite acidified with acetic acid is added to the mixture of the four cations (the anion present is nitrate). A precipitate is produced. The aqueous solution is decanted off of the precipitate into test tube 12. What is the formula for the precipitate in test tube 112 Step 2. Aqueous ammonia is added to the solution in test tube 12 until the solution is basic. A precipitate is produced. The aqueous solution is decanted off of the precipitate into test tube 3. What is the formula for the precipitate in test tube 12? Step 3. An ethanol solution of dimethylglyoxime is added to the solution in test tube 13. A precipitate is produced. The aqueous solution is decanted off of the precipitate into test tube 14. What is the formula for the precipitate in test tube 13? What is the formula for the transition metal cation in test tube ? Step 4. To check that indeed there is a final transition metal cation in the solution in test tube 14, another sample of the original mixture of all four cations is tested with aqueous potassium iodide. A precipitate forms. What is the formula for this confirming precipitate?

Answer 1

In step 1) test tube t1

The precipitate formed is of yellow colour. This precipitate is of Co²⁺ ion. formula: K₃[Co(NO₂)₆]

Net ionic equation :

CO2+ +6NO, → CO(NO2)61-3

the final oxidation state of cobalt is Co³⁺

in step 2)

The ion that gets precipitated in t2 is Fe³⁺. The colour of the ppt is gelatinous reddish-brown. and the formula of the precipitate is Fe(OH)₃

Net ionic equation

Fe3+ + 3N HAOH → Fe(OH)3 + 3NH

Note: Aqueous ammonia's formula is NH₄OH

in step 3)

The precipitate formed in t3 is of Ni⁺² ions. The formula for precipitate is Ni[DMG]₂ or (C₄H₇N₂O₂)₂Ni. The Color of this ppt is scarlet red.

Net ionic equation

Ni+2 + 2C4H7N202 + (C4H7N202)2 Ni

Now since, Co²⁺, Fe³⁺ and Ni⁺² have been precipitated in t1, t2 and t3 respectively, the only transition metal cation left in t4 is Cu²⁺ ions.

So t4 solution contains Cu²⁺ ions.

in step 4)

The only cation that reacts with potassium iodide is Cu²⁺ ions.

The Cu⁺² precipitates and gives a white colour ppt. The formula for this ppt is Cu₂I₂.

Net ionic equation :

204+2 +51 → Cuzl2 + 13

[Note: The solution is intensely brown due to the presence of tri-iodide ion, but the solution becomes colourless and a white precipitate is visible when excess of sodium thiosulphate solution is added. ]

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