Question

In a metal oxide, there is 20% oxygen by weight, then equivalent weight is:
A.$64$
B.$32$
C.$72$
D.$40$

Answer 1

Hint: Percentage of an element in the compound is calculated as the ratio of mass of that element in the compound to the total mass of the compound multiplied by $100$. Total mass of the compound is calculated as the sum of the masses of all elements present in the compound.

Complete step by step solution:
First of all let us read about molecular formulas.
Molecular weight of the substance: It is defined as the sum of the molecular masses of all elements present in the substance.
Number of moles: It is defined as the ratio of mass of the substance to the molecular mass of the substance.
Molecular formula: The molecular composition in the compound is known as molecular formula. For example: the molecule of water is ${H_2}O$ i.e. two atoms of hydrogen with oxygen.
Now here we are given the percentage of oxygen in its metal oxide. Let the mass of the metal be $x$ and we know that the mass of oxygen molecules is $32$ .
Now here we are given the percentage of oxygen in the compound.
As $100g$ metal oxide contains $20gm$ of oxygen then $32gm$ of metal oxide will contain $\dfrac{{32 \times 20}}{{100}} = 6.4gm$ of oxygen. Now as the weight of oxygen is $20\% $ so the metal will be $80\% $ by weight. Hence we can say the weight of metal will be $4$ times that of the weight of oxygen. Here the weight of oxygen is $6.4gm$ so the weight of metal will be $4 \times 6.4 = 25.6gm$. So the equivalent weight (i.e. the total weight) of the metal oxide will be $25.6 + 6.4 = 32$.

Hence, option B is correct.

Note:
Formula mass: It is defined as the sum of the atomic weights of all the atoms present in the compound. And if the compound is covalent in nature then the formula mass is known as molecular mass and if we are calculating mass of one mole of substance then it is known as molar mass.