Some Basic Concepts of Chemistry 2 Question 34
34. An aqueous solution containing $0.10 \mathrm{~g} \mathrm{KIO}_{3}$
$($ formula weight $=214.0)$ was treated with an excess of KI solution. The solution was acidified with $\mathrm{HCl}$. The liberated $\mathrm{I}_{2}$ consumed $45.0 \mathrm{~mL}$ of thiosulphate solution decolourise the blue starch-iodine complex. Calculate the molarity of the sodium thiosulphate solution.
$(1998,5 \mathrm{M})$
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Solution:
- The redox reaction involved are
$$ \begin{aligned} \mathrm{IO}{3}^{-}+5 \mathrm{I}^{-}+6 \mathrm{H}^{+} & \longrightarrow 3 \mathrm{I}{2}+3 \mathrm{H}{2} \mathrm{O} \ \mathrm{I}{2}+2 \mathrm{~S}{2} \mathrm{O}{3}^{2-} & \longrightarrow 2 \mathrm{I}^{-}+\mathrm{S}{4} \mathrm{O}{6}^{2-} \end{aligned} $$
millimol of $\mathrm{KIO}_{3}$ used $=\frac{0.1}{214} \times 1000=0.467$
$\Rightarrow$ millimol of $\mathrm{I}_{2}$ formed $=3 \times 0.467=1.4$
$\Rightarrow$ millimol of $\mathrm{Na}{2} \mathrm{~S}{2} \mathrm{O}_{3}$ consumed $=2 \times 1.4=2.8$
$\Rightarrow$ Molarity of $\mathrm{Na}{2} \mathrm{~S}{2} \mathrm{O}_{3}=\frac{2.8}{45}=0.062 \mathrm{M}$
