Question

The number of moles of KI required to produce $0.1$mole of ${{K}_{2}}Hg{{I}_{4}}$is?
A. $1.6$
B. $0.8$
C. $3.2$
D. $0.4$

Answer 1

The question is based on the mole concept. For solving such a type of question, we need to first write a balanced chemical equation and then use the simple and basic stoichiometric calculations to find out the required number of moles, atoms etc.

Complete step-by-step answer: ${{K}_{2}}Hg{{I}_{4}}$is also known as Nessler’s reagent. Nessler’s reagent is the alkaline solution of the complex formed by dissolving mercuric Chloride in aqueous solution of potassium iodide useful to detect $N{{H}_{3}}$.
The balanced chemical equation for the formation of Nessler’s reagent is as follows:
$HgC{{l}_{2}}+4KI\to 2KCl+{{K}_{2}}Hg{{I}_{4}}$
From the above reaction we can clearly see that $4$moles of potassium iodide$\left( KI \right)$are required to produce $1$ mole of Nessler’s reagent$\left( {{K}_{2}}Hg{{I}_{4}} \right)$
Or the above statement can also be written as follows,
$1$ mole of ${{K}_{2}}Hg{{I}_{4}}$requires = $4$moles of $KI$ to get formed
In question we are asked to find the number of moles of $KI$ require to produce $0.1$mole of ${{K}_{2}}Hg{{I}_{4}}$
Therefore, using unitary method we can say that if
$1$ mole of ${{K}_{2}}Hg{{I}_{4}}$require = $4$moles of potassium iodide $KI$ to get formed
Therefore, $0.1$mole of ${{K}_{2}}Hg{{I}_{4}}$requires,
$0.1\times 4=0.4$moles of $KI$ to get formed.
Hence, $0.4$moles of Potassium iodide are required.
Therefore, the correct option is D. $0.4$

Note: It should be noted that Nessler’s reagent crystallizes with either one, two or three molecules of water. It is commercially available in anhydrous form. It is soluble in water and in general is denser than water. In such a type of question, always write a balanced chemical equation and always determine the starting and ending point to solve the problems related to the mole concept.