A gaseous mixture of $\mathrm{O}_2$ and $\mathrm{CH}_4$ are in the ratio $1: 4$ by mass. Find the ratio of their molecules.

Calculate the relative lowering of vapour pressure of solution containing 3 g urea in 50 g water. [molar mass of urea $=60 \mathrm{~g} \mathrm{~mol}^{-1}$ ]

Identify the type of defect in brass alloy.

What is the difference in molar masses of Undecane and Decane?