Question

A compound made up of two elements $X$ and $Y$ has an empirical formula $X_{2}Y$. If the atomic weight of $X$ is $10$ and that of $Y$ is $5$ and the compound has a vapour density $25$, find its molecular formula.

Answer 1

Emp. formula $=X_{2}Y$, At. wt. of $X=10$, At. wt. of $Y=5$
$\therefore$ Empirical formula mass $=2\times 10+5=25$
If Vapour density $V.D.=25$
Mol. Mass $=V.D.\times 2=25\times 2=50u$
$n=\frac{Mol.Mass}{Emp.formulamass}=\frac{50}{25}=2$
Mol. formula $=Emp.formula\times 2=X_{2}Y\times 2$
$=X_4{Y}_2$.