00:01
In the problem number 75, we need to calculate the freezing point and melting point, or probably what they meant is the boiling point of a solution, of actually a benzene solution containing some naftali.
00:17
And in order to calculate both of the freezing depression point and the melting depression point or boiling depression point, we need to make use of the van -hoff equation.
00:39
So that's the basic thing that we need to keep in mind in order to be able to solve this problem.
00:52
And the vanhoff equations as that the delta tf, which is the shift of the freezing point, is equal to molality, malality of the solution times the kf factor.
01:17
And an analog equation exists for the boiling point depression.
01:27
Again, malality times kb, which is a different benthoff factor.
01:39
So this is what we need to calculate in order to be able to determine the final temperature which will be t final is equal to t0 plus or minus delta t okay so let's see what we're given we know both of the factors in this equation so kf from the book for nafzile, no, sorry, for benzene, is equal to 5 .12 degrees celsius divided by the malality unit.
02:44
And kb is equal to 2 .53 celsius divided by the malality factor.
02:54
So this we already know.
02:56
We also know the freezing temperature and the boiling temperature of pure benzene.
03:06
So t .f.
03:10
Pure is equal to 5 .5 degrees celsius.
03:19
And the boiling temperature is equal to 80 .1 degrees celsius.
03:28
Now we know that our solution consists of 100 milliliters of benzene, so volume of benzene 100 milliliters.
03:51
We also know the density benzene is equal to 0 .877 grams per cubic centimeter, which is the same as grams per milliliter.
04:13
We also know that we have mass of naphtylene dissolved in the solution.
04:18
So mass of the salute is equal to 10 grams.
04:38
So now that we see that half equation, we know that one value that we still need to determine is the malality.
04:48
And the molality is given by this equation.
05:01
It's the amount of salute divided by the mass of solvent, not the solution, very important.
05:14
So now we can calculate the amount of the salute, which is naphthalene, from the mass that we're given, and the mass of solvent from the volume that we're given...