Question

A bivalent metal atom has $12$ equivalent weight. Find the molecular weight of its oxide.
$(i){\text{ 16}}$
$(ii){\text{ 32}}$
$(iii){\text{ 40}}$
$(iv){\text{ 52}}$

Answer 1

We are given with the equivalent weight of the binary metal element. We will find its $n -$factor. With the help of $n -$factor we will find its molecular weight. The oxide formed by the bivalent metal looks like$XO$, where X is a bivalent metal. Then we will find its molecular weight.
Formula Used:
${\text{Equivalent weight = }}\dfrac{{{\text{weight of element}}}}{{n - factor}}$

Complete Answer:
Bivalent metal atom is one which has a valency of $+ 2$. We can also say the bivalent metals are those which can form ionic bonds with two atoms. Thus they have a tendency to form two bonds. This is why they are called bivalent metal atoms. Such metal atoms which show valency of $+ 2$ and form two ionic bonds are metal atoms of group $II$ of periodic table.
Firstly we will find the weight of the given metal atom. We know that,
${\text{Equivalent weight = }}\dfrac{{{\text{weight of element}}}}{{n - factor}}$
For bivalent atom, the value of $n -$factor is two. And the equivalent weight of metal atom is given which is $12$. The weight of metal atom can be found as,
${\text{12 = }}\dfrac{{{\text{weight of element}}}}{2}$
${\text{weight of element = 12 }} \times {\text{ 2}}$
${\text{weight of element = 24 g}}$
Thus the metal atom of group $II$ of the periodic table whose weight is $24{\text{ g}}$ is Magnesium. The oxide formed by magnesium is,
$= {\text{ 40 g}}$ $Mg{\text{ + }}{{\text{O}}_2}{\text{ }} \to {\text{ MgO}}$
Thus its oxide is magnesium oxide. The molecular weight of magnesium can be calculated as,
Molecular weight of oxide $= {\text{ }}\left( {{\text{24 + 16}}} \right){\text{ g}}$
Molecular weight of oxide $= {\text{ 40 g}}$
Thus the molecular weight of the oxide is ${\text{40 g}}$.

Hence the correct option is $(iii){\text{ 40}}$.

Note:
A bivalent atom makes two bonds with other atoms. Also we can say that the number of valence electrons in a bivalent atom is two. Since it is a metal therefore it is likely to donate the pair of electrons. The chemical equation is not a balanced chemical equation since our major concern is to get formula for oxide.