Question

$10mL$ of a potassium dichromate solution liberates iodine from potassium iodide solution. When the iodine was titrated with hypo solution $\left(N/20\right)$, the titre value was $15mL$. Find the concentration of dichromate solution in $g$ per $litre$.

Answer 1

The reactions involved may be given as:
$C{r}_2{O}_7^{2-}+6I^{-}+14H^{+}\to 2C{r}^{3+}+3I_2+7H_{2}O$
$3[I_2+2N{a}_2{S}_2{O}_3]\to 3[2NaI+N{a}_2{S}_4{O}_6]$
$1mole{K}_{2}C{r}_2{O}_7\equiv 6moles$ of $N{a}_2{S}_2{O}_3....\left(i\right)$
No. of moles of hypo $=\frac{Mass}{M.w.\left(158\right)}=\frac{E\times N\times V}{1000\times 158}$
$=\frac{158\times 1/20\times 15}{1000\times 158}$
$=7.5\times {10}^{-4}mole$
No. of moles of $K_{2}C{r}_2{O}_7=\frac{1}{6}$ [No. of moles of $N{a}_2{S}_2{O}_3]$
$=1.25\times {10}^{-4}mole$
Mass of $K_{2}C{r}_2{O}_7$ in $10mL$ solution $=1.25\times {10}^{-4}\times 294$
$=0.03675g$
Weight of $K_{2}C{r}_2{O}_7$ in $10mL$ solution $=3.675g/L$.