COD of water sample is 8ppm the weight of acidified $K_{2}C{r}_2{O}_7$ required to oxidise the organic matter present in one litre of water sample is _________ mg

8ppm means, ${10}^{6}g$ of $H_{2}O$ contain 8gm of $O_2$ then 1lt=1000ml=1000g of $H_{2}O$ contain $8{\text{ x 10}}^{-3}g$ of  $O_2$

8gm of $O_2$ required 49g of  $K_{2}C{r}_2{O}_7$ then $8{\text{ x 10}}^{-3}$  g of $O_2$ required 

$\begin{array}{l}\frac{49{\text{x8x10}}^{-3}}{8}=49{\text{x10}}^{-3}g\\ =49mg\end{array}$