Question

The equivalent weight $HN{O_3}$ in the following reaction is:
$3Cu + 8HN{O_3} \to 3Cu{\left( {N{O_3}} \right)_2} + 2NO + 4{H_2}O$
(A) $63$
(B) $189$
(C) $126$
(D) $84$

Answer 1

The equivalent weight of the element is defined as the quantity or the mass of the substance that reacts in the reaction. The equivalent weight of an element in a chemical reaction is equal to the ratio of the gram atomic weight of the element to the valency of the element.
Formula Used:
$E = \dfrac{w}{n}$ where $E$ is the equivalent weight of a chemical element, $w$ is the gram atomic weight of the element, and $n$ is the valency of the element.

Complete step by step answer:
Now we are in a condition to solve the question as we know the basic concept of equivalent weight. So first, we will study the given reaction and discuss the chemistry behind it. So let’s start with the reaction $3Cu + 8HN{O_3} \to 3Cu{\left( {N{O_3}} \right)_2} + 2NO + 4{H_2}O$ . In the given reaction we can observe that three moles of copper react with eight moles of nitric acid $HN{O_3}$ to give six moles of copper nitride and nitrogen monoxide. So we can express it as $8HN{O_3}$ gives $6NO_3^ -$ and $2NO$ . Now we will observe the change in oxidation states of nitrogen to find the valency. In the given reaction we can observe that the oxidation state of nitrogen in $HN{O_3}$ is $+ 5$ and the oxidation state of nitrogen in $Cu{\left( {N{O_3}} \right)_2}$ is $+ 2$ and the oxidation state of nitrogen in $NO$ is $+ 2$ . So the total transfer of electrons or the valency $2NO$ will be $n = 3 + 3 = 6$ . Now to calculate the equivalent of the molar mass or the gram atomic weight of $HN{O_3}$ is $63g/mol$ . Now the equivalent weight for $8$ moles of $HN{O_3}$ will be $E = \dfrac{w}{n} = \dfrac{{63}}{6} \times 8 = 84$ .
Therefore, the correct option is (D).

Note:
There is a difference between the equivalent weight and molecular weight. Molecular weight is the sum of the atomic weight of each constituent element multiplied by the number of atoms in the formula unit. Whereas equivalent weight is the mass that will combine or be displaced directly.