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Standard XII

Chemistry

Oxidising Reagent and Reducing Reagent

12 g of an im...

Question

$12 g$ of an impure sample of arsenious oxide was dissolved in water containing $7.5 g$ of sodium bicarbonate and the resulting solution was diluted to $250 m L$ . $25 m L$ of this solution was completely oxidised by $22.4 m L$ of a solution of iodine. $25 m L$ of this iodine solution reacted with same volume of a solution containing $24.8 g$ of sodium thiosulphate solution $(N a_{2} S_{2} O_{3} \cdot 5 H_{2} O)$ in one litre. Calculate the percentage of arsenious oxide in the sample. (Atomic mass of $A s = 75)$ .

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Solution

Normality of $N a_{2} S_{2} O_{3} s o l n . = \frac{24.8}{248} = 0.1 N$
Applying $N_{1} V_{1} = N_{2} V_{2}$
Volume of $A s_{2} O_{3}$ soln. in $N a H C O_{3} \times$ Normality of this soln.
$=$ Volume of iodine soln. $\times$ Normality of iodine soln.
$25 \times N_{1} = 22.4 \times 0.1$
$∴ N_{1} = \frac{22.4 \times 0.1}{25}$
Amt. of $A s_{2} O_{3}$ present in $250 m L$ of the solution
$= N_{1} = \frac{E q u i v a l e n t m a s s o f A s_{2} O_{3}}{1000} \times 250$
$= \frac{22.4 \times 0.1}{25} \times \frac{198}{4} \times \frac{250}{1000} = 1.1088 g$
Percentage of $A s_{2} O_{3} = \frac{1.1088}{12} \times 100 = 9.24$ .

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