When an element forms an oxide in which oxygen is $20\% $ of the oxide by mass, the equivalent mass of the element will be:
A) $32$
B) $40$
C) $60$
D) $128$

Hint: To solve this question, we must first understand the concept of equivalent mass. Then we need to assess the concept and the given data to evaluate the equivalent mass of oxygen and then only we can conclude the correct answer.


Complete solution:
Before we move forward with the solution of this given question, let us first understand some basic concepts about equivalent weight:
Equivalent Weight: It is the number of parts or the mass of a substance which combines/reacts/displaces $1.008$ parts by mass of Hydrogen or $8$ parts by mass of Oxygen or $35.5$ parts by mass of Chlorine is called its equivalent mass. And also quantity of a substance in grams numerically equals to its equivalent mass is called its gam equivalent mass or gram equivalent.
Step 1: In this step we will enlist all the given quantities:
Given, Oxygen content present in the given oxide $ = 20\% $
So, according to the question $100g$ of oxide contains $20g$ of oxygen and $80g$ of element.
Step 1: In this step we will calculate the equivalent mass of the given element:
Equivalent mass of element $ = \,\dfrac{{mass\,\,of\,element}}{{mass\,\,of\,oxygen}} \times 8$ $ = \dfrac{{80}}{{20}} \times 8 = $ $32$
So, the required equivalent mass is $32g$

Therefore, the correct answer is (A).


Note:For acid-base reactions, the equivalent weight of an acid or base is the mass which supplies or reacts with one mole of hydrogen cations $({H^ + })$ . For redox reactions, the equivalent weight of each reactant supplies or reacts with one mole of electrons $({e^ - })$ in a redox reaction. Additionally, the equivalent weight of a compound can be calculated by dividing the molecular mass by the number of positive or negative electrical charges that result from the dissolution of the compound.