Chapter 19, Problem 19.115QP

In an analytical determination of arsenic, a solution containing arsenious acid, H₃AsO₃, potassium iodide, and a small amount of starch is electrolyzed. The electrolysis produces free iodine from iodide ion, and the iodine immediately oxidizes the arsenious acid to hydrogen arsenate ion, HAsO₄²⁻.

$\begin{array}{l}{\text{I}}_2(aq)+{\text{H}}_3{\text{AsO}}_3(aq)+{\text{H}}_2\text{O}(l)\stackrel{}{\to }\\ 2{\text{I}}^{-}(aq)+{\text{HAsO}}_4{}^{2-}(aq)+4{\text{H}}^{+}(aq)\end{array}$

When the oxidation of arsenic is complete, the free iodine combines with the starch to give a deep blue color. If, during a particular run, it takes 65.4 s for a current of 10.5 mA to give an endpoint (indicated by the blue color), how many grams of arsenic are present in the solution?