Question

The equivalent weight of ${\text{KI}}{{\text{O}}_3}$ in the given reaction is:
 \[{\text{2Cr}}{\left( {{\text{OH}}} \right)_{\text{3}}}{\text{ + O}}{{\text{H}}^{\text{ - }}}{\text{ + KI}}{{\text{O}}_{\text{3}}} \to {\text{Cr}}{{\text{O}}_{\text{4}}}^{{\text{2 - }}}{\text{ + 5}}{{\text{H}}_{\text{2}}}{\text{O + KI}}\]

A.Molecular Weight
B.$\dfrac{{{\text{molecular weight}}}}{{\text{3}}}$
C.$\dfrac{{{\text{molecular weight}}}}{6}$
D.$\dfrac{{{\text{molecular weight}}}}{2}$

Answer 1

Hint: The equivalent weight of an oxidant is equal to the ratio of the molecular weight of the oxidant to the number of electrons gained by the molecule. The equivalent weight of an acid or a base is the number of replaceable protons or the number of replaceable hydroxyl ions.

Complete Stepwise Solution:
In the above reaction, the chromium in the chromium hydroxide is being oxidized by the iodine of potassium iodate which itself gets reduced by accepting electrons from the chromium ion to form potassium iodide. The oxidation state of chromium in chromium hydroxide is $ + 3$, because it pairs with three monovalent hydroxyl groups to form the hydroxide. The oxidation state of iodine in potassium iodate is $ + 5$. The chromium donates three electrons to the iodate under basic conditions to form the chromate anion, in which the oxidation state of chromium is $ + 6$, hence it gets oxidized while the iodine atom in iodate accepts three electrons from the chromium to form potassium iodide in which the oxidation state of iodine is $ - 1$.
So the number of electrons transferred in the process is three, hence the equivalent weight of ${\text{KI}}{{\text{O}}_3}$ in the given reaction is :
$\dfrac{{{\text{molecular weight}}}}{{\text{3}}}$= $\dfrac{{39 + 127 + \left( {16 \times 3} \right)}}{3} = \dfrac{{224}}{3} = 74.66$

So, the correct answer is option C.

Notes: Just like the equivalent weight or and oxidant or reductant is the ratio of the molecular weight to the number of electrons gained or lost, the equivalent weight of an acid or a base is the number of replaceable protons or the number of replaceable hydroxyl ions, respectively.