In an analytical determination of aresenic , a solution containing aresenious acid, `(H_(3)AsO_(3))`. `KI` and a small amount of starch is electrolysed. The electrolysis produes free `I_(2)` from `I^(-)` ion and the `I_(2)` immediately oxidises the arsenious acid to hydrogen arsenate ion, `(HAsO_(4)^(2-))`.
`I_(2(aq.))+H_(3)AsO_(3(aq.))+H_(2)O_((l))rarr2I_((aq.))^(-)+HAsO_(4(aq.))^(2-)+4H_((aq.))^(+)`
When the oxidation of arsenic is complete, the free iodine combines with the starch to give a deep blue colour. If during a particular run, it takes `65.3 s` for a current of `10.5 mA` to give an end point (indicated by the blue colour), how many grams of arsenic and `H_(3)AsO_(3)` are present in the solution? `("At. wt. of "As = 75)`

Answer

As `= 2.66 xx 10^(-4)g`,
`H_(3)AsO_(3) = 5.0 xx 10^(-4)g ;`