Question

The oxide of an element possess the molecular formula $M_2{O}_3$. If the equivalent mass of the metal is $9$, the molecular mass of the oxide will be :

• A. $27$
• B. $75$
• C. $102$
• D. $18$

Answer 1

The correct option is C $102$

Let M be the molar mass of the element.

2M g of element combines with $3\times 16=48g$ of oxygen.

$\frac{2M\times 8}{48}=\frac{M}{3}$ g of element combines with $8g$ of oxygen.

The equivalent mass of an element is the mass of the element that combines with 8 g of oxygen.

Thus, the equivalent mass of the element is $\frac{M}{3}$ . But it is equal to 9.

Hence, $M=3\times 9=27g/mol$

The molecular mass of the metal oxide is $2\left(27\right)+3\left(16\right)=102g/mol$.