A flask contains hydrogen and oxygen in the ratio of $2 : 1$ by mass at temperature $27 °\mathrm{C}$. The ratio of average kinetic energy per molecule of hydrogen and oxygen respectively is :

Average Kinetic Energy per molecule $K{E}_{avg}=\frac{3}{2}{k}_{B}T$. Therefore, we can conclude that the average kinetic energy of the molecules is directly proportional to the temperature of the gas and is independent of pressure, volume or the nature of the gas.

Hence, required ratio $=1:1$