In basic medium ${CrO}_4^{2-}$ oxidises $S_2{O}_3^{2-}$ to form ${SO}_4^{2-}$ and itself changes into $Cr(OH{)}_4^{-}$. The volume of $0.154M{CrO}_4^{2-}$ required to react with $40 mL$ of $0.25M{S}_2{O}_3^{2-}$ is $mL$ (Rounded off to the nearest integer)

 

Given, Molarity of ${CrO}_4^{-2}=0.154M$

Volume of $S_2{O}_3^{-2}==40ml$

Molarity of $S_2{O}_3^{-2}==0.25M$

 Let the volume of ${CrO}_4^{-2}=V$

Normality of ${CrO}_4^{-2}=30.154$

Normality of $S_2{O}_3^{-2}==80.25$

$N_1{V}_1=N_2{V}_2$

$V\times 0.154\times 3=0.25\times 8\times 40$

V = 173ml

Therefore, volume of ${CrO}_4^{-2}$ is 173ml.