Question

What will be the colour of the solution when $Mn{(OH)_2}$ is treated with concentrated $HN{O_3}$ and sodium bismuthate (or red lead or lead dioxide) ?
a.) Yellow
b.) Purple
c.) Green
d.) Blue

Answer 1

When the Manganese (II) ions reacts with nitric acid in the presence of sodium bismuthate permanganate ions are produced. The reaction is as -
$2M{n^{2 + }} + 5NaBi{O_3} + 14{H^ + } \to 2MnO_4^ - + 5B{i^{3 + }} + 5N{a^ + } + 7{H_2}O$
The colour of the solution is the colour of permanganate ions which are present in solution.

Complete answer:
First, let us see the reaction of $Mn{(OH)_2}$ with concentrated $HN{O_3}$and sodium bismuthate.
When the Manganese (II) ions react with nitric acid in the presence of sodium bismuthate, a violet or purple colour product is formed. This purple colour is because of the presence of permanganate ions. The reaction is as -
$2M{n^{2 + }} + 5NaBi{O_3} + 14{H^ + } \to 2MnO_4^ - + 5B{i^{3 + }} + 5N{a^ + } + 7{H_2}O$
So, the colour is purple.

Thus, the option b.) is the correct answer.

Additional information:
The permanganate ions have a wide range of applications in industries. These are used for oxidation processes, in cleaning liquids etc. The potassium permanganate is used as disinfectant due to oxidising properties. Further, it is also used for medication of dermatitis and for cleaning wounds. There are many other uses.

Note:
It must be noted that in permanganate, the Mn is in +7 oxidation state. It is a very strong oxidising agent. These permanent solutions are stable in neutral and alkaline environments. There are many other ways to form permanganate ions. The most common is the oxidation of Mn ions. The colour is due to charge transfer transitions.