Question

An unknown inorganic compound $\left(X\right)$ loses its water of crystallisation. On heating its aqueous solution gives the following reacion:
(i) it gives a white turbidity with dilute $HCl$ solution.
(ii) it decolourises a solution of iodine in $KI$.
(iii) it gives a white precipitate with $AgN{O}_3$ solution which turns black on standing.
Identify compound $\left(X\right)$?

• A. $X=N{a}_2{S}_2{O}_3.4H_{2}O$
• B. $X=N{a}_2{S}_2{O}_3.5H_{2}O$
• C. $X=N{a}_2{S}_2{O}_3.6H_{2}O$
• D. none of these

Answer 1

Answer: (B)

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

Since the compound $X$ decolourize a solution of iodine in $KI$, it should contain thiosulphate ion, $S_2{O}_3^{2-}$ which also gives white turbidity with $HCl$ and white precipitate with $AgN{O}_3$. Hence the $X$ is Sodium thiosulphate $N{a}_2{S}_2{O}_3.5H_{2}O$.

$N{a}_2{S}_2{O}_3.5H_{2}O\stackrel{Heat}{⟶}N{a}_2{S}_2{O}_3+5H_{2}O\uparrow$

$N{a}_2{S}_2{O}_3+HCl\to 2NaCl+H_{2}O+S{O}_2+S\left(whiteturbidity\right)$

$N{a}_2{S}_2{O}_3+I_2\to N{a}_2{S}_4{O}_6+2NaI$

$N{a}_2{S}_2{O}_3+AgN{O}_3\to A{g}_2{S}_2{O}_3\left(Whiteppt\right)\downarrow +NaN{O}_3$

$A{g}_2{S}_2{O}_3+H_{2}O\to A{g}_{2}S\left(Blackppt\right)+H_{2}S{O}_4$