Question

A compound contains 3.2 % of oxygen. The minimum molecular weight of the compound is:
A ) 300
B ) 440
C ) 350
D ) 500

Answer 1

Hint: Mass percent of oxygen in the compound is one hundred times the ratio of the mass of oxygen present in one molecule of the compound to the mass of one molecule of the compound. If only one oxygen atom is present in one molecule of compound, then, the mass percent of oxygen in the compound is also equal to one hundred times the ratio of the atomic weight of oxygen present in the compound to the molecular weight of the compound.
$$\text{Mass percent of oxygen = 100 }\times {\displaystyle \frac{\text{Atomic weight of oxygen}}{\text{Molecular weight of the compound}}}$$

Complete answer:
Let $$Mg/mol$$ be the minimum molecular weight of the compound. The minimum number of oxygen atoms present in the compound is one. The atomic weight of oxygen is $$16g/mol$$ . Divide the atomic weight of oxygen with the minimum molecular weight of the compound, and multiply this ratio with 100, to obtain the mass percent of oxygen in the compound.
$$\begin{gathered} \text{Mass percent of oxygen = 100 }\times {\displaystyle \frac{16\text{ g/mol}}{M\text{ g/mol}}} \\ ={\displaystyle \frac{1600}{M}} \end{gathered}$$
But, the compound contains 3.2 % of oxygen.
$$\begin{gathered} \text{Mass percent of oxygen = 3}\text{.2\% } \\ ={\displaystyle \frac{1600}{M}} \end{gathered}$$
Hence,
$$\begin{gathered} 3.2={\displaystyle \frac{1600}{M}} \\ M={\displaystyle \frac{1600}{3.2}} \\ M=500\text{ g/mol} \end{gathered}$$
The minimum molecular weight of the compound is $$500g/mol$$ .

Hence, the option D ) 500 is the right answer.

Note: During the calculation for the mass percent of oxygen, use the atomic weight of oxygen as 16 g/mol. Do not use molecular weight of oxygen as 32 g/mol. This is because in the chemical formula, the minimum value of suffix for oxygen is one, which represents one atom of oxygen and not one molecule of oxygen.