Question

The equivalent mass of $KI{{O}_{3}}$ in the reaction
$2Cr{{(OH)}_{3}}+O{{H}^{-}}+KI{{O}_{3}}\to 2Cr{{O}_{4}}^{2-}+KI+5{{H}_{2}}O$ is:
(A) M
(B) $\dfrac{M}{3}$
(C) $\dfrac{M}{6}$
(D) $\dfrac{M}{2}$

Answer 1

Hint: Atoms combine to form chemical compounds, all elements in that compound always present in definite proportions by mass. This property can be used to make chemically different with a different mass.

Complete step by step answer:
Definition of equivalent mass: all elements combine according to the laws of chemical combination, and the number of parts by which an element combines with 1 gram equivalent of any element, is the value of the equivalent mass of the element.

Equivalent mass is based on the valency of the element. The formula to calculate the equivalent mass of an element given by,
\[Equivalent\text{ }mass=\dfrac{\text{Molar }mass}{valency}\]
There are methods for determining the equivalent mass of compounds. The methods are
The equivalent mass formula for acids = $\dfrac{molecular\text{ }mass\text{ }of\text{ }acid}{basicity}$
The equivalent mass formula for bases = $\dfrac{molecular\text{ }mass\text{ }of\text{ base}}{acidity}$

The balanced chemical reaction is,
$2Cr{{(OH)}_{3}}+O{{H}^{-}}+KI{{O}_{3}}\to 2Cr{{O}_{4}}^{2-}+KI+5{{H}_{2}}O$
The oxidation number of iodine changes from +5 to -1.
The change in the oxidation number of iodine = 6
The equivalent mass of potassium iodate = $\dfrac{M}{6}$
Where M = molar mass of potassium iodate.
Hence, The equivalent mass of $KI{{O}_{3}}$ the reaction is $\dfrac{M}{6}$.
So, the correct answer is “Option C”.

Note: If we cannot find the valency, there are some methods to determine the equivalent mass of an element. The three most important methods are the hydrogen displacement method, oxide method, and the chloride method. The hydrogen displacement method is used to calculate equivalent mass under standard temperature and pressure.