Question

The formula of oxide of a metal is ${\text{MO}}$ and the equivalent weight of the metal is 20. Find the atomic weight of the metal:
A.40
B.20
C.30
D.10

Answer 1

The term atomic weight of an element is used to refer to the average relative mass of an atom of that element as compared to the weight of ${{\text{C}}^{{\text{12}}}}$ isotope which is taken as 12 units. On the other hand, the equivalent weight of an element refers to the mass of the substance which can combine or displace a fixed amount of another substance. It is related to the atomic weight of the element by the valency.

Complete step by step answer:
Given that the formula of the oxide of a metal is ${\text{MO}}$.
Also given that the equivalent weight of the metal is 20.
We need to calculate the atomic weight of the metal.
Since the atomic weight is related to the equivalent weight through valency, let us first understand what valency means.
In chemistry, the term valency or valence of an element refers to the combining power of the element with other atoms during the formation of molecules or chemical compounds. According to the IUPAC definition, valency is said to be the maximum number of univalent atoms which combines with an atom of a given element. Alternatively, it represents the number of hydrogen atoms which can combine with a given element in a binary hydride or twice the number of oxygen atoms which can combine with the given element in its oxide.
Now, the equivalent weight values are actually the atomic weight values divided by the valency. Or, it can be said that the atomic weight is equal to the equivalent weight multiplied by the valency. Thus,
${\text{AtomicWeight = EquivalentWeight \ Valency}}$
Since the number of oxygen atoms in the given metal oxide is 1, therefore the valency of the metal will be twice the number of oxygen atoms, i.e., 2.
Now, the atomic weight of the metal can be calculated by substituting the values of equivalent weight and valency of the metal.
So , the atomic weight of the given metal
$= 20 \times 2 \\\ = 40 \\\$
So, option A is correct.

Note:
The equivalent weight can be determined from the molecular weight of the substance divided by the number of positive or negative charges resulting from its dissolution. For an acid, equivalent weight is the molecular weight of the acid divided by the basicity of the acid and for a base, equivalent weight is the molecular weight of the base divided by the acidity of the base. For a salt, equivalent weight is the molecular weight divided by the sum of the valencies of the ions.