Question

A compound contains $6.4\%$ by mass of oxygen. It contains two moles of oxygen atoms per mole of the compound. What is the molar mass of the compound?

• A. $500{\text{g mol}}^{-1}$
• B. $525{\text{g mol}}^{-1}$
• C. $333.33{\text{g mol}}^{-1}$
• D. $125{\text{g mol}}^{-1}$

Answer 1

The correct option is A $500{\text{g mol}}^{-1}$

If we take the basis as $100\text{g}$ of the compound, $6.4\%$ by mass of oxygen means that it contains $6.4g$ of oxygen.

$2$ oxygen atoms are present in $1$ molecule of the compound. Then, $32\text{g}$ of oxygen will be present in one gram mole of the compound.
Let molar mass of the compound be $M$.
$\therefore$ $\frac{6.4}{100}\times M=32{\text{g mol}}^{-1}$
$\Rightarrow M=500{\text{g mol}}^{-1}$