Question

A compound has the following composition: Na=18.6%, S=25.8%, H=4.03% and O=51.58%. Calculate the molecular formula of the crystalline salt assuming that all the hydrogen in the compound is in combination with the oxygen as water of crystallisation. Molecular weight of the compound is 248.

• A. $N{a}_2{S}_2{O}_3.5H_{2}O$
• B. $Na{S}_2{O}_5.3H_{2}O$
• C. $N{a}_2{S}_3{O}_5.3H_{2}O$
• D. $N{a}_2{S}_4{O}_3.5H_{2}O$

Answer 1

Answer: (A)

$N{a}_2{S}_2{O}_3.5H_{2}O$

Element Relative number of atoms Simplest ratio of whole numbers

Sodium $\frac{18.6}{23}=0.8$ $\frac{0.8}{0.8}=1$

Sulphur $\frac{25.8}{32}=0.8$ $\frac{0.8}{0.8}=1$

Hydrogen $\frac{4.03}{1}=4.03$ $\frac{4.03}{0.8}=5$

Oxygen $\frac{51.58}{16}=3.22$ $\frac{3.22}{0.8}=4$

Empirical formula $=NaS{H}_5{O}_4$

Molecular weight$=248$

Empirical formula weight$=23+32+5+64=124$

n=$\frac{248}{124}=2$

Molecular formula=$NaS{H}_5{O}_4\times 2=N{a}_2{S}_2{H}_{10}{O}_8$

Since, all the hydrogen in the compound is in combination with the oxygen as water of crystallization so 10 atoms of hydrogen and 5 atoms of oxygen.

So, the formula becomes $N{a}_2{S}_2{O}_3.5H_{2}O$.

Option A is correct.