Chapter ER, Problem 1PQ

Interpretation Introduction

Interpretation:

The pair which has the same oxidation number has to be predicted.

Concept Introduction:

Oxidation number:

The oxidation of an atom is the charge that atom would have if the compound was composed of ions.  The oxidation number of an atom depends on the other atoms in the substance. For example,

• In $\text{KCl}$ the oxidation number of Cl is 0.
• In $\text{Cl}₂\text{,}$ the oxidation number of Cl is $\text{-1}\text{.}$

Answer to Problem 1PQ

The oxidation number of sulfur is same in ${\text{H}}_{\text{2}}{\text{S}}_{\text{2}}{\text{O}}_{\text{7}}$ and ${\text{H}}_{\text{2}}{\text{SO}}_{\text{4}}.$  Thus the option (A) is correct.

Explanation of Solution

Reason for the correct answer:

The oxidation number of hydrogen is one and the oxidation number of oxygen is $\text{-2}$ the oxidation number of sulfur is taken as x.

The oxidation number of sulfur in ${\text{H}}_{\text{2}}{\text{S}}_{\text{2}}{\text{O}}_{\text{7}}$ is calculated as,

  $\begin{array}{l}\text{2+2x-14=0}\\ \text{2x=12}\\ \text{}\text{}\text{}\text{}\text{x=6}\end{array}$

The oxidation number of sulfur in ${\text{H}}_{\text{2}}{\text{SO}}_{\text{4}}$ is calculated as,

  $\begin{array}{l}\text{2+x-8=0}\\ \text{}\text{}\text{}\text{x=6}\end{array}$

The oxidation number of sulfur in ${\text{H}}_{\text{2}}{\text{S}}_{\text{2}}{\text{O}}_{\text{7}}$ and ${\text{H}}_{\text{2}}{\text{SO}}_{\text{4}}$ is six.  Hence, option (A) is correct.

Reasons for incorrect answer:

The oxidation number of ${\text{Hg}}^{\text{2+}}$ is two whereas the oxidation number of ${\text{Hg}}_{\text{2}}{}^{\text{2+}}$ is one.  Hence, option (B) is incorrect.

The oxidation number of oxygen in ${\text{Na}}_{\text{2}}{\text{O}}_{\text{2}}$ is calculated as,

  $\begin{array}{l}\text{2-2x=0}\\ \text{}\text{}\text{}\text{}\text{x=1}\end{array}$

The oxidation number of oxygen in ${\text{H}}_{\text{2}}\text{O}$ is calculated as,

  $\begin{array}{l}\text{2-x=0}\\ \text{}\text{}\text{}\text{}\text{x=-2}\end{array}$

The oxidation number of oxygen in ${\text{Na}}_{\text{2}}{\text{O}}_{\text{2}}$ is $\text{1}$ whereas the oxidation number of oxygen in ${\text{H}}_{\text{2}}\text{O}$ is $\text{-2}\text{.}$  Hence, option (C) is incorrect.

The oxidation number of cobalt in $\text{Co}{\left({\text{NH}}_{\text{3}}\right)}_{\text{6}}{}^{\text{3+}}$ is calculated as,

Since ${\text{NH}}_{\text{3}}$ is neutral ligand its oxidation number must be zero.

  $\begin{array}{l}\text{x+0=+3}\\ \text{}\text{}\text{}\text{}\text{x=+3}\end{array}$

The oxidation number of cobalt in $\text{Co}{\left({\text{NO}}_{\text{3}}\right)}_{\text{2}}$ is calculated as,

The oxidation number of ${\text{NO}}_{\text{3}}$ is $\text{-1}\text{.}$

  $\begin{array}{l}\text{x-2=0}\\ \text{}\text{}\text{}\text{}\text{x=2}\end{array}$

The oxidation number of cobalt in complexes $\text{Co}{\left({\text{NH}}_{\text{3}}\right)}_{\text{6}}{}^{\text{3+}}$ and $\text{Co}{\left({\text{NO}}_{\text{3}}\right)}_{\text{2}}$ is three and two respectively.  Hence, option (D) is incorrect.