Question

A certain mass of a substance, when dissolved in 100 g of $C_6{H}_6$, lowers the freezing point by ${1.28}^{o}C$.The same mass of solute dissolved in 100 g of water lowers the freezing point by ${1.40}^{o}C$.If the substance has normal molecular weight in benzene and is completely dissociated in water, into how many ions does it dissociate in water?$K_f$ for $H_{2}O$ and $C_6{H}_6$ is 1.86 and $5.12K{mol}^{-1}kg$, respectively:

Answer 1

In benzene,$\Delta T_f=K_{f}m$
$1.28=5.12\times m$
$\therefore m=0.25$
In $H_{2}O$, ionization takes place ,
$\therefore \Delta T_f=i{K}_{f}m$
$1.40=i\times 1.86\times 0.25$
$\therefore i=3$