A mixture of gases contains $H_2$ and $O_2$ gases in the ratio of 1:4 by weight. What is the molar ratio of the two gases in the mixture?

The correct option is B 4:1
Number of moles (n) $=\frac{\text{given mass(m)}}{\text{molar mass (M)}}$
As the ratio by weight of Hydrogen and Oxygen is 1:4,
Molar ratio will be $\frac{n_{H_2}}{n_{O_2}}=\frac{\frac{1}{2}}{\frac{4}{32}}=\frac{32}{8}$ = 4:1
The molar ratio of Hydrogen and Oxygen is 4:1.