Oxidation number of O in ${ H }_{ 2 }{ O }_{ 2 }$ is?
A) 1-
B) 0
C) 2+
D) 3+

Hint: We should know that the oxidation number is the formal charge that an atom appears to have when the electrons are counted as per certain arbitrary ​rules. Now you can try calculating the oxidation number of O in hydrogen peroxide.

Complete step by step answer:

You need to assign oxidation numbers to the elements in a compound by using the rules for oxidation numbers.
The important rules for this problem are:
Rule 1 - The oxidation number of H is +1, but it is -1 in when combined with less electronegative elements.
Rule 2 - The oxidation number of O in compounds is usually -2, but it is -1 in peroxides.
Rule 3 - The sum of the oxidation numbers of all the atoms in a neutral compound is 0.
The oxidation number of H is +1 (Rule 1). If you know that ${ H }_{ 2 }{ O }_{ 2 }$ is hydrogen peroxide, you can immediately assign oxygen to the oxidation number = -1.
If you don't know that is ${ H }_{ 2 }{ O }_{ 2 }$ hydrogen peroxide, you can use rule 3 and the oxidation number of H is +1. For two H atoms, the sum is +2. Since ${ H }_{ 2 }{ O }_{ 2 }$ has no charge, the total oxidation number of the two O atoms must be -2.
The oxidation number of H is +1. The oxidation number of O is -1.
Hence, the oxidation number of one oxygen atom must be -1.
Therefore the correct answer to this question is option A.

Note: You should also know some more facts about hydrogen peroxide -
Hydrogen peroxide is the simplest kind of peroxide available (oxygen-oxygen single bond).
It is a colorless liquid and is used in aqueous solution for safety reasons.
It acts as a bleaching agent and is also used as a disinfectant.
Concentrated hydrogen peroxide is a very reactive oxygen species and is used as a propellant in rocketry.