Chapter 21, Problem 104A

(a)

Interpretation Introduction

Interpretation: The oxidation number of each atom in ${\text{CaCr}}_{\text{2}}{\text{O}}_{\text{7}}$ needs to be determined.

Concept Introduction: An oxidation number of an element is defined as the number assigned to it in any chemical composition representing number of electrons gained or lost by an atom of that element in forming the chemical compound.

Explanation of Solution

The given substance is ${\text{CaCr}}_{\text{2}}{\text{O}}_{\text{7}}$ . This can be represented as follow:

  ${\text{Ca}}^{2+}$ and ${\text{Cr}}_{\text{2}}{\text{O}}_7^{2-}$

Thus, oxidation number of Ca is +2. Now, general oxidation number of oxygen is -2 thus, oxidation number of Cr is calculated as follows:

  $\begin{array}{l}2x+7\left(-2\right)=-2\\ 2x-14=-2\\ 2x=-2+14\\ 2x=12\\ x=6\end{array}$

Therefore, oxidation number of Cr is +6.

The oxidation number of Ca, Cr and O atom is +2, +6 and -2 respectively.

(b)

Interpretation Introduction

Interpretation: The oxidation number of each atom in ${\text{KMnO}}_{\text{4}}$ needs to be determined.

Concept Introduction: An oxidation number of an element is defined as the number assigned to it in any chemical composition representing number of electrons gained or lost by an atom of that element in forming the chemical compound.

Explanation of Solution

The given substance is ${\text{KMnO}}_{\text{4}}$ . This can be represented as follows:

  ${\text{K}}^{+}$ and ${\text{MnO}}_4^{-}$

Thus, the oxidation number of K is +1. Now, general oxidation number of O is -2 thus, oxidation number of Mn is calculated as follows:

  $\begin{array}{c}x+4\left(-2\right)=-1\\ x-8=-1\\ x=-1+8\\ x=+7\end{array}$

Therefore, oxidation number of Mn is +7.

The oxidation number of K, Mn and O is +1, +7 and -2 respectively.

(c)

Interpretation Introduction

Interpretation: The oxidation number of each atom in $\text{Ca}{\left({\text{NO}}_{\text{3}}\right)}_{\text{2}}$ needs to be determined.

Concept Introduction: An oxidation number of an element is defined as the number assigned to it in any chemical composition representing number of electrons gained or lost by an atom of that element in forming the chemical compound.

Explanation of Solution

The given substance is $\text{Ca}{\left({\text{NO}}_{\text{3}}\right)}_{\text{2}}$

This can be represented as ${\text{Ca}}^{2+}$ and ${\text{NO}}_3^{2-}$

Thus, the oxidation number of Ca is +2. The general oxidation number of O is -2. Thus, the oxidation number of N can be calculated as follows:

  $\begin{array}{c}x+3\left(-2\right)=-2\\ x-6=-2\\ x=-2+6\\ x=+4\end{array}$

Thus, oxidation number of N is +4.

The oxidation number of Ca, N and O is +2, +4 and -2 respectively.

(d)

Interpretation Introduction

Interpretation: The oxidation number of each atom in $\text{Al}{\left(\text{OH}\right)}_{\text{3}}$ needs to be determined.

Concept Introduction: An oxidation number of an element is defined as the number assigned to it in any chemical composition representing number of electrons gained or lost by an atom of that element in forming the chemical compound.

Explanation of Solution

The given substance is $\text{Al}{\left(\text{OH}\right)}_{\text{3}}$

This can be represented as follows:

  ${\mathrm{Al}}^{3+}$ and $3{\text{OH}}^{-}$

Thus, oxidation number of Al is +3. The general oxidation number of O and H is -2 and 1 respectively.

Therefore, the oxidation number of Al, O and H is +3, -2 and +1 respectively.