Elements $P$ and $Q$ form two types of non-volatile, non-ionizable compounds $P Q$ and $P Q_2$. When 1 g of PQ is dissolved in 50 g of solvent ' $\mathrm{A}, \Delta \mathrm{T}_{\mathrm{b}}$ was 1.176 K while when 1 g of $\mathrm{PQ}_2$ is dissolved in 50 g of solvent ' $\mathrm{A}, \Delta \mathrm{T}_{\mathrm{b}}$ was $0,689 \mathrm{~K}\left(\mathrm{~K}_{\mathrm{b}}\right.$ of ' $\left.\mathrm{A}=5 \mathrm{~K} \mathrm{~kg} \mathrm{~mol}^{-1}\right)$. The molar masses of elements P and Q (in gmol $^{-1}$ ) respectively, are :