c
\((I)\,\mathop {SeO_3^{2 - }}\limits^{ + 4}  + \mathop {BrO_3^ - }\limits^{ + 5}  + {H^ + } \to \mathop {SeO_4^{2 - }}\limits^{ + 6}  + \mathop {B{r_2}}\limits^0  + {H_2}O\)

\((II)\,\mathop {BrO_3^{- }}\limits^{ + 5}  + \mathop {AsO_2^ - }\limits^{ + 3}  + {H_2}O \to \mathop {B{r^ - }}\limits^{ - 1}  + \mathop {AsO_4^{3 - }}\limits^{ + 5}  + {H^ + }\)

In reaction \((II)\)

gm. eq. of \(BrO_3^-=\) gm. eq. of \(AsO_2^-\)

\(n_{BrO_3^-} \times 6=n_{AsO_2^-} \times 2\)

\( = \frac{{12.5}}{{1000}} \times \frac{1}{{25}} \times 2 = {10^{ - 3}}\)

\({n_{BrO_3^ - }} = \frac{{{{10}^{ - 3}}}}{6}\)

In reaction \((I)\)

moles of \(BrO_3^-\) consumed

\( = \frac{{70}}{{1000}} \times \frac{1}{{60}} - \frac{{{{10}^{ - 3}}}}{6} = {10^{ - 3}}\)

gm eq. of \(SeO_3^{2-} = gm. eq. of BrO_3^-\)

\(n_{SeO_3^{2-}} \times 2 = 10^{-3} \times 5\) ;

\(n_{SeO_3^{2-}} = 2.5 \times 10^{-3}\)