# Which of the following has maximum freezing point for 1 molal solution, assuming equal ionisation in each case?

$\begin{array}{1 1}(a)\;[Fe(H_2O)_3Cl_3].3H_2O\\(b)\;[Fe(H_2O)_4Cl_2].Cl.2H_2O\\(c)\;[Fe(H_2O)_5Cl].Cl_2.H_2O\\(d)\;[Fe(H_2O)_6].Cl_3\end{array}$

Since, Maximum freezing point $\propto \large\frac{1}{\text{No of dissociated particles}}$
Since, the compounds $[Fe(H_2O)_4Cl_2].Cl.2H_2O, [Fe(H_2O)_5Cl].Cl_2.H_2O$ and $[Fe(H_2O)_6].Cl_3$ has $Cl^-$ as a ligand not attached to the central atom, they dissociate into its constituents.
$[Fe(H_2O)_3Cl_3].3H_2O$ is a single species, because there is no such metal or non-metal not attached to the central atom. So, i = 0
Thus, $[Fe(H_2O)_3Cl_3].3H_2O$ has the maximum freezing point.
Hence (a) is the correct answer.
edited Jul 20, 2014