Question

The oxidation state of oxygen of ${{\text{H}}_2}{\text{O}}$ in the final products when it reacts with ${\text{A}}{{\text{s}}_2}{{\text{O}}_3}$ is:
A. 0
B. 1
C. -1
D. -2

Answer 1

First determine the products of the reaction. Then identify the product which contains oxygen atoms. Then write the oxidation numbers of all atoms (except oxygen) present in this product. Then calculate the oxidation number of oxygen atoms.

Complete Step by step answer:
The chemical symbol ${{\text{H}}_2}{\text{O}}$ represents water. The chemical symbol ${\text{A}}{{\text{s}}_2}{{\text{O}}_3}$ represents the element arsenic oxide.
Water reacts with arsenic oxide to form arsenic element and hydrogen peroxide. Write the unbalanced chemical equation for the reaction between water and arsenic oxide to obtain arsenic and hydrogen peroxide.
${{\text{H}}_2}{\text{O}} + {\text{A}}{{\text{s}}_2}{{\text{O}}_3} \to {{\text{H}}_2}{{\text{O}}_2} + {\text{As}}$
Write the balanced chemical equation for the reaction between water and arsenic oxide to obtain arsenic and hydrogen peroxide.
${\text{3}}{{\text{H}}_2}{\text{O}} + {\text{A}}{{\text{s}}_2}{{\text{O}}_3} \to 3{{\text{H}}_2}{{\text{O}}_2} + 2{\text{As}}$
Three moles of water react with one mole of arsenic oxide to obtain three moles of hydrogen peroxide and two moles of arsenic.
In the final products, the oxygen atom is present in hydrogen peroxide molecules.
Let x be the oxidation state of an oxygen atom in a hydrogen peroxide molecule. The oxidation state of the hydrogen atom is +1.
In a neutral molecule, the sum of the oxidation states of all the atoms is equal to zero. Hence, in the hydrogen peroxide molecule the sum of the oxidation numbers of two hydrogen atoms and two oxygen atoms is zero.

$$2\left( { + 1} \right) + 2x = 0 \\\ \Rightarrow 2 + 2x = 0 \\\ \Rightarrow 1 + x = 0 \\\ \Rightarrow x = - 1 \\\$$

Hence, the oxidation number of oxygen atoms in the final product, hydrogen peroxide is $- 1$.

Hence, the correct option is the option C.

Note: The reaction between water and arsenic oxide to obtain arsenic and hydrogen peroxide is a redox reaction. In this reaction, water is oxidized as it gains an oxygen atom. Arsenic oxide is reduced as it loses oxygen atoms.