Solutions and Colligative Properties 2 Question 12
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12. For 1 molal aqueous solution of the following compounds, which one will show the highest freezing point?(2018 Main)
======= ####12. For 1 molal aqueous solution of the following compounds, which one will show the highest freezing point?(2018 Main)
3e0f7ab6f6a50373c3f2dbda6ca2533482a77bed (a) $\left[\mathrm{Co}\left(\mathrm{H}{2} \mathrm{O}\right){6}\right] \mathrm{Cl}_{3}$
(b) $\left[\mathrm{Co}\left(\mathrm{H}{2} \mathrm{O}\right){5} \mathrm{Cl}^{2} \mathrm{Cl}{2} \cdot \mathrm{H}{2} \mathrm{O}\right.$
(c) $\left[\mathrm{Co}\left(\mathrm{H}{2} \mathrm{O}\right){4} \mathrm{Cl}{2}\right] \mathrm{Cl} \cdot 2 \mathrm{H}{2} \mathrm{O}$
(d) $\left[\mathrm{Co}\left(\mathrm{H}{2} \mathrm{O}\right){3} \mathrm{Cl}{3}\right] \cdot 3 \mathrm{H}{2} \mathrm{O}$
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Solution:
- Key idea “Addition of solute particles to a pure solvent results to depression in its freezing point.”
All the compounds given in question are ionic in nature so, consider their van’t Hoff factor $(i)$ to reach at final conclusion.
The solution with maximum freezing point must have minimum number of solute particles. This generalisation can be done with the help of van’t Hoff factor $(i)$ i.e.
Number of solute particles $\propto$ van’t Hoff factor $(i)$
Thus, we can say directly
Solution with maximum freezing point will be the one in which solute with minimum van’t Hoff factor is present
Now, for $\left.\mathrm{Co}\left(\mathrm{H}{2} \mathrm{O}\right){6}\right] \mathrm{Cl}{3} \rightleftharpoons\left[\mathrm{Co}\left(\mathrm{H}{2} \mathrm{O}\right)_{6}\right]^{3+}+3 \mathrm{Cl}^{-}$
van’t Hoff factor $(i)$ is 4 . Similarly for,
$\left[\mathrm{Co}\left(\mathrm{H}{2} \mathrm{O}\right){5} \mathrm{Cl}\right] \mathrm{Cl}{2} \cdot \mathrm{H}{2} \mathrm{O} \rightleftharpoons\left[\mathrm{Co}\left(\mathrm{H}{2} \mathrm{O}\right){5} \mathrm{Cl}^{2+}+2 \mathrm{Cl}^{-}\right.$’ $i$ ’ is 3
$\left[\mathrm{Co}\left(\mathrm{H}{2} \mathrm{O}\right){4} \mathrm{Cl}{2}\right] \mathrm{Cl} \cdot 2 \mathrm{H}{2} \mathrm{O} \rightleftharpoons\left[\mathrm{Co}\left(\mathrm{H}{2} \mathrm{O}\right){4} \mathrm{Cl}_{2}\right]^{+}+\mathrm{Cl}^{-}$’ $i$ ’ is 2
and for $\left[\mathrm{Co}\left(\mathrm{H}{2} \mathrm{O}\right){3} \mathrm{Cl}{3}\right] \cdot 3 \mathrm{H}{2} \mathrm{O}$, ’ $i$ ’ is 1 as it does not show ionisation. Hence, $\left[\mathrm{Co}\left(\mathrm{H}{2} \mathrm{O}\right){3} \mathrm{Cl}{3}\right] \cdot 3 \mathrm{H}{2} \mathrm{O}$ have minimum number of particles in the solution.
So, freezing point of its solution will be maximum.
