The titrations

Exercise 3

       

$25\;ml$ of iodine water (solution of iodine in water) are titrated by a $0.05\;M$ solution of sodium thiosulfate. The iodine is completely discolored after addition of $8.0 \; ml$ $Na_2S_2O_3$. Calculate the mass of iodine in these $25\; ml$ of initial solution knowing that the essential equation starts like this : $I_2$ $+$ $2S_2O_3^{2-}$ $\longrightarrow$ $..$

$n_{S_2O_3^{2-}}$ $=$ $V_{S_2O_3^{2-}} \cdot [S_2O_3^{2-}]$ $=$ $0.008 \cdot 0.05 $ mol One mole $S_2O_3^{2-}$ reacts with $\frac{1}{2}$ mole $I_2$ $n_{I_2}$ $=$ $\frac{1}{2}\cdot 0.008 \cdot 0.05$ mol $m_{I_2}$ $=$ $n\cdot M$ $=$ $\frac{1}{2}\cdot 0.008 \cdot 0.05\cdot 253.8$ $=$ $0.051g$