Q. A compound has the following composition by weight $: Na = 18.60\%, S = 25.80\%, H = 4.02\%$ and $O = 51.58\%.$ Assuming that all the hydrogen atoms in the compound are part of water of crystallisation, the correct molecular formula of the compound is
KVPYKVPY 2017Some Basic Concepts of Chemistry
Solution:
Elements
$\%$ of Elements
At mass of element
Moles of element
Simplest molar ratio
Simplest whole no
$Na$
$18.6$
$23$
$\frac{18.6}{23} = 0.8$
$\frac{0.8}{0.8} = 1$
$1 \times 2 =2$
$S$
$25.8$
$32$
$\frac{25.8}{32} = 0.8$
$\frac{0.8}{0.8}=1$
$1 \times 2 =2$
$O$
$51.58$
$16$
$\frac{51.58}{16} =3.22$
$\frac{3.22}{0.8} =4$
$4 \times 2=8$
$H$
$4.02$
$1$
$\frac{4.02}{1} =4.02$
$\frac{4.02}{0.8}=5$
$5 \times 2=10$
Thus, the empirical formula of compound is $Na_{2}S_{2}H_{10}O_{8}$. As it is given the all the hydrogen atoms in the compound are part of water of crystallisation, therefore molecular formula will be $Na_{2}S_{2}O_{3} \cdot 5H_{2}O$
| Elements | $\%$ of Elements | At mass of element | Moles of element | Simplest molar ratio | Simplest whole no |
|---|---|---|---|---|---|
| $Na$ | $18.6$ | $23$ | $\frac{18.6}{23} = 0.8$ | $\frac{0.8}{0.8} = 1$ | $1 \times 2 =2$ |
| $S$ | $25.8$ | $32$ | $\frac{25.8}{32} = 0.8$ | $\frac{0.8}{0.8}=1$ | $1 \times 2 =2$ |
| $O$ | $51.58$ | $16$ | $\frac{51.58}{16} =3.22$ | $\frac{3.22}{0.8} =4$ | $4 \times 2=8$ |
| $H$ | $4.02$ | $1$ | $\frac{4.02}{1} =4.02$ | $\frac{4.02}{0.8}=5$ | $5 \times 2=10$ |