When 79.75 g of  $CuS{O}_4$ sample containing impurity is reacted with KI, the liberated  $I_2$ is reacted with 50 mL (1M) $N{a}_2{S}_2{O}_3$  in basic medium, where it oxidized into  $S{O}_4^{-2}$  ions, and   $I_2$reduces into $I^{-}$   , then what will be the % purity of  $CuS{O}_4$ in sample?