Question

$P$ and $Q$ are two elements which form $P_2{Q}_3$ and $P{Q}_2$ compounds. If $0.15$ moles of $P_2{Q}_3$ & $P{Q}_2$ weigh $15.9$ g and $9.3$ g respectively, then the atomic mass of $6Q$ is $Y$ . Then $Q$ is:

Answer 1

Suppose atomic masses of $P$ and $Q$ are $X$ and $Y$ respectively.

Molar mass of $P_2{Q}_3=2X+3Y$

and Molar mass of $P{Q}_2=X+2Y$

$\therefore 2X+3Y=\frac{15.9}{0.15}....\left(i\right)$ and $X+2Y=\frac{9.3}{0.15}....\left(ii\right)$ (Moles = Weight/Molecular weight)

Solving these equations, $X=26g{mol}^{-1},Y=18g{mol}^{-1}$

So, the atomic mass of $6Q$ is $18$.

Hence, $Q=3$.