Question

Which would have a greater impact on the freezing point depression of ice, CrF3 or CaF2? Explain briefly.

Answer 1

Answer: $CrF_3$ has greater impact on the freezing point depression of ice.

Explanation:

Depression in freezing point is given by:

$\Delta T_f=i\times K_f\times m$

$\Delta T_f=T_f-T_f^0$ = Depression in freezing point

i= vant hoff factor

$K_f$ = freezing point constant

m= molality

$\Delta T_f=i\times K_f\times \frac{\text{mass of solute}}{\text{molar mass of solute}\times \text{weight of solvent in kg}}$

a) i = 4 for $CrF_3$ as it dissociates to give 4 ions in water.

$CrF_3\rightarrow Cr^{3+}+3F^-$

b) i = 3 for $CaF_2$ as it dissociates to give 3 ions in water.

$CaF_2\rightarrow Ca^{2+}+2F^-$

As the vant hoff factor is higher for $CrF_3$ , it has greater impact on the freezing point depression of ice.