The goal here was to determine the molar mass (in g/mole) of camphor by measuring how much the addition of a known mass of camphor depressed the freezing point of a known mass of pure cyclohexane.
In the Results section, you should have a table similar to the following (these are not real data:)
Table 1. Experimental Data.
| Trial | Mass of Cyclohexane (g) | Freezing point of solvent (oC) | Mass Camphor (g) | Freezing point of solution (oC) |
| 1 | 5.20 | 6.4 | 0.27 | -0.2 |
| 2 | 5.25 | 6.4 | 0.25 | -0.1 |
Recall that the freezing points were obtained from Logger Pro during the experiment. You measured the freezing point of pure cyclohexane only once (you're assuming it didn't change from Trial 1 to Trial 2.)
Your report should contain three plots of your cooling curves (one for the pure solvent and two for the camphor trials.) Import the data into Excel and make plots similar to the following:
Now for the calculations: here, we make a sample calculation for trial 1 data in the table above. Repeat this calculation for the trial 2 data and report the average molar mass of the camphor.
We have DTf = kfm,
Where DTf = Freezing point of pure solvent - freezing point of solution
kf = 20.00 oC kg /mol
m = molality = (g camphor/MM camphor) / kg solvent
From trial 1 above, DTf = (6.4 - (-0.2)oC = 6.6oC
We'll do this step-by-step: first,
DTf / kf = m, so
6.6oC / 20.00 oC kg/mol = m = 0.33 mol/kg
so the molality = 0.33 mol/kg. Now, m = (g camphor/MM camphor) / kg solvent. Multiplying by kg solvent (from Table 1),
0.33 mol/kg (0.0052 kg) = (g camphor/MM camphor), so
0.00172 mol = g camphor/MM camphor
from trial 1, table 1, we have 0.27 g, so
0.00172 mol = 0.27 g / MM camphor
Solving for MM camphor,
MM camphor = 0.27 g / 0.00172 mol = 157 g/mol.
Repeat this calculation for your second trial, and report the average molar mass.
