`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 mL`. `25 mL` of this solution was completely oxidised by `22.4 mL` of a solution of iodine. `25 mL` of this iodine solution reacted with same volume of a solution containing `24.8 g` of sodium thiosulphate `(Na_(2)S_(2)O_(3).5H_(2)O)` in one litre. Calculate teh percentage of arsenious oxide in the sample ( Atomic mass of `As=74`)

Explanation

Normality of `Na_(2)S_(2)O_(3)` soln. `=(24.8)/(248)=0.1N`
Applying `N_(1)V_(1)=N_(2)V_(2)`
Volume of `As_(2)O_(3)` soln. in `NaHCO_(3)xx` Normality of this soln.
=Volume of iodine soln. `xx ` Normality of iodine soln.
`25xxN_(1)=22.4xx0.1`
`therefore " " N_(1)=(22.4xx0.1)/(25)`
Amt. of `As_(2)O_(3)` present in 250 mL of the solution
`N_(1)xx ("Equivalent mass of " As_(2)O_(3))/(1000)xx250`
`=(22.4xx0.1)/(25)xx(198)/(4)xx(250)/(1000)=1.1088 g`
Percentage of `As_(2)O_(3)=(1.1088)/(12)xx100=9.24`