A sample containing `HAsO_(2)` (mol. Mass=108) and weighing 3.78 g is dissolved and diluted to 250 mL in a volumetric flask. A 50 mL sample (aliquot) is withdrawn with a pipet and titrated with 25 mL of 0.05 M solution of `I_(2)`. Calculate the percentage `HAsO_(2)` in the sample :

Options

A. `25%`
B. `20%`
C. `10%`
D. `15%`

Answer

A

Explanation

`I_(2)+ AsO_(2)^(-) + 2H_(2)O to HAsO_(4)^(2-) +2I^(-) +3H^(+)`
m-eq. of `HAsO_(2)` (in 50 mL)=m-eq. of `I_(2) =35 xx 0.05 xx 2= 3.5`
m-eq. of `HAsO_(2)` in 250mL=`3.5 xx (250)/(50) = 17.5`
mass of `HAsO_(2)` in sample= `(17.59/(2) xx (108) xx 10^(-3))`
=0.945 g
% of `HAsO_(2)` in the sample= `(0.945)/(3.78) xx 100 = 25%`