I did an experiment to determine the freezing point depression and then find the molecular weight of the unknown solute added to the solvent, what would be the effect on calculating the molecular weight if some cyclohexane(my solvent) evaporated while the freezing point of the pure substance was being measured and also the effect if some cyclohexane evapoated after the solute was added?

Question

I am thinking since we use the formula delta T=K(f)*molality to get the molality(m) and then from there calculate the no of mole using the formula m= moles/kg of sovent, since the kg of sovent would change so would our final ans...is this right??

Answer 1

of course it changes, that is a no brainer. But how does this affect the mol mass determined?

tempdepression=k masssolute/(molmass*kgsolvent)

molmass= k masssolute/kg solvent ( 1/temp)

so what happens to the mol mass if kg solvent goes down?

Answer 2

The mol mass calculated would be higher than expected.
but when we are measuring the pure cyclohexane's freezing point, this wouldn't matter right?? The freezing point would be the same no matter what the volume of solvent.

Answer 3

Thanks!