Question

When ammonia hydroxide is added to solution $B$ , a pale blue precipitate is formed. This pale blue precipitate dissolves in excess ammonium hydroxide giving an inky blue solution. What is the cation [positive ion] present in solution $B$ ? What is the probable colour of solution $B$ .

Answer 1

Hint: When ammonia hydroxide reacts with a solution of copper sulphate and copper hydroxide is produced in the product which is blue pale in colour. Copper hydroxide is having $C{u^{2 + }}$ cations in it.

Complete answer:
In this question, the chemical formula of ammonia hydroxide can be written as $N{H_4}OH$ . Ammonia hydroxide is a dark blue solution. As it is given that when this ammonia hydroxide $N{H_4}OH$ reacts with a solution $B$ , then a pale blue precipitate is formed. As we know that when we react ammonia hydroxide $N{H_4}OH$ with the solution of copper sulphate $CuS{O_4}$ which is having chemical formula as then the product obtained is then copper hydroxide $Cu{(OH)_2}$ and copper sulphate is obtained in the product.
This reaction can be represented as follows:
$2N{H_4}OH + CuS{O_4} \to Cu{(OH)_2} + {(N{H_4})_2}S{O_4}$
                                          [Pale blue]
Here, we can see that in this reaction the solution $B$ is Copper sulphate $CuS{O_4}$ solution and in the product, it is copper hydroxide $Cu{(OH)_2}$ that is a pale blue coloured precipitate.
In the second part of the question, it is given that when the above formed pale blue precipitate reacts with excess ammonium hydroxide then it gives an inky blue solution. Therefore, it means that when precipitate of Copper hydroxide $Cu{(OH)_2}$ reacts with excess ammonium hydroxide then it forms tetra amine copper $[II]$ sulphate which is a deep blue soluble complex salt and have the chemical formulas as $[Cu{(N{H_3})_4}]S{O_4}$ .
This reaction can be represented as:
$Cu{(OH)_2} + {(N{H_4})_2}S{O_4} + 2N{H_4}OH \to [Cu{(N{H_3})_4}]S{O_4} + 4H{ _2}O$
Therefore, the required solution $B$ is copper hydroxide $Cu{(OH)_2}$ and also, we know that in copper hydroxide the copper has $ + 2$ charge. Therefore, copper sulphate has $C{u^{2 + }}$ cation in it.

Note:
Ammonia hydroxide $N{H_4}OH$ is a dark blue coloured compound and when it reacts with copper sulphate $CuS{O_4}$ then it converts the colour of the solution into pale blue in the form of copper hydroxide $Cu{(OH)_2}$ .