Question

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 crystallization, the correct molecular formula of the compound is:

• A. $N{a}_2{S}_2{O}_3\cdot 3H_{2}O$
• B. $N{a}_{2}S{O}_4\cdot 5H_{2}O$
• C. $N{a}_{2}S{O}_4\cdot 10H_{2}O$
• D. $N{a}_2{S}_2{O}_3\cdot 5H_{2}O$

Answer 1

Answer: (D)

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

We assume that there are 100g of sodium salt.

$Na:\frac{18.6}{23}=0.8S:\frac{25.8}{32}=0.8H:\frac{4.02}{1}=4O:\frac{51.58}{16}=3.2$

To get the empirical formula we divide by lowest number (0.4)

We will get our formula $N{a}_2{S}_2{O}_3.5H_{2}O$.