What are the differences in measuring the molar mass using freezing point depression between studying ionic solids and molecular solids?

Question

aaronq · Answer

Molecular solids do not ionize into several particles while ionic solids do (to some extent). Since Freezing-point depression is a colligative property, caused by the presence of particles of non-volatile solute (and not on their chemical nature), ionic solids have a multiplicative effect on the change in temperature per unit - i.e. the van't hoff constant, $i$.

FP depression equation:  $\large \Delta T=i*\underbrace{m}_{molality}*K_b$

Molecular solids have $i$=1 always, and ionic solids $i\geq2. $

aaronq · Answer

i forgot to mention that you would rearrange the formula to find the molar mass, like this:

$\Delta T=i*m*K_b$

sub in $m=\dfrac{n_{solute}}{kg_{solvent}}$

$\Delta T=i*(\dfrac{n_{solute}}{kg_{solvent}})*K_b$

sub in $n=\dfrac{m}{M}$

$\Delta T=i*(\dfrac{\dfrac{m}{M}}{kg_{solvent}})*K_b$

Then you get: $\Delta T=\dfrac{i*m*K_b}{kg_{solvent}*M}$

Rearrange for molar mass, M:  $M=\dfrac{i*m*K_b}{kg_{solvent}*\Delta T}$

aaronq · Answer

NOTE the first $m$ is molality and the second $m$ is mass.