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?
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??
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?
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.
Thanks!
Yes, you are correct. In order to determine the molecular weight of the unknown solute, you used the freezing point depression method which involves measuring the change in freezing point of the solvent (in this case, cyclohexane) after adding the solute.
If some cyclohexane evaporated while measuring the freezing point of the pure substance, it would lead to an overestimation of the freezing point depression. This can affect the accuracy of your calculated molality. Remember that the molality is calculated by dividing the moles of solute by the mass of the solvent (in kilograms). If the mass of the solvent decreases, you would calculate a higher molality value, which would then result in a higher estimate for the molecular weight of the solute.
Similarly, if some cyclohexane evaporated after adding the solute, it would also affect the accuracy of your calculated molality. The decrease in the mass of the solvent would lead to an overestimated molality, and thus a higher estimate for the molecular weight of the solute.
To mitigate the impact of solvent evaporation, it is crucial to ensure accurate measurements of the mass of the solvent before and after the experiment. Minimizing the time and exposure to external factors that could cause evaporation, such as sealing the container or conducting the experiment in a controlled environment, can also help improve the accuracy of your results.
It is important to be aware of these potential sources of error and make efforts to minimize them in order to obtain more accurate molecular weight calculations.