Cryoscopic Determination of Molar Mass
By: Stephanie McKinney
Partners: Billy and Chanel
Experiment Completed: 11-19-09
The experimental molecular mass for naphthalene was determined by adding .6g, .9g, and 1.2g
of the substance into a solvent of cyclohexane and graphically determining the freezing point. Once the
freezing points of the naphthalene were determined to be 2.93±.07 οC, 1.10±.05 οC, -.75±.07 οC
respectively, the respective molar mass values were able to be calculated. The molar mass for the
various amounts of naphthalene were determined to be be 168±5 g/mol, 170±2 g/mol, and 170±4 g/mol
This equation was used in order to determine the density of the cyclohexane because it was
temperature dependant, where the variable t was room temperature.
This equation was used to determine the experimental value for the molar mass of naphthalene
where g is the mass of the solute, G is the mass of the solvent, Kf is the molal freezing-point depression
constant, and ΔTf is the difference in the literature value freezing point of cyclohexane and the average
freezing points of the various naphthalene value.
This equation was also used to determine the experimental value for the molar mass of
naphthalene where the only different variables were kf which is the correction constant factor.
These values were used in coordination with Eq 1 in order to determine the density and
eventually the mass of the cyclohexane used.
Table 1: Determination of Density and Mass of Cyclohexane
Room Temp Volume
ο Density g/cm3 Mass g
24.05±.05 0.77477±.00005 25.0±.5 19.4±.4
These values were the freezing points of the cyclohexane and naphthalene at the various
masses. They were graphically determined in Figure 1-Figure 8 by plotting the temperature versus time
of each trial.
Table 2: Freezing Points of Cyclohexane and Naphthalene
Trial 1 οC Trial 2 οC Average οC
Cyclohexane 4.10±.05 4.15±.05 4.13±.07
.6g Naphthalene 2.80±.05 3.05±.05 2.93±.07
.9g Naphthalene 1.10±.05 1.10±.05 1.10±.05
1.2g Naphthalene -.70±.05 -.80±.05 -.75±.07
These values were determined using Eq 2 and Eq 3 to determine the experimental mass of the
naphthalene at .6g, .9g, and 1.2g. Presented is the literature value of naphthalene along with the
Table 3;Experimental Molecular Mass of Naphthalene
Molar Mass Molar Mass
Literature(g/mol) % Deviation
.6g Naphthalene 168±5 167±6 128.17 31±3%
.9g Naphthalene 170±2 169±8 128.17 33±2
1.2g Naphthalene 170 ±4 170± 20 128.17 33±3%
The experimental molar masses for .6g, .9g, and 1.2g of naphthalene using Eq 2 were
determined to be 168±5, 170±2, and 170±4 grams per mol respectively. With the correction value for
the molal freezing point depression constant, Eq 3, the molecular weight of naphthalene was calculated
to be 167±6, 169±8, and 170± 20. When comparing the literature value of the naphthalene to those
values calculated from Eq 2 the percent deviations were determined to be 31±3%, 33±2%, and 33±3%
respectively. Once the molar mass values were determined using Eq 2 they were then plotted against
their respective average freezing point of 2.93±.07 οC, 1.10±.05 οC, -.75±.07 οC. A trend line was then
extrapolated back to the y-axis to determine the graphical molar mass of 169.9±.6 grams per mole.
When this value was compared to that of the literature value a percent deviation of 32.6±.7% was
calculated. The most accurate molar mass value was that of the .6g of naphthalene and the least
accurate was that of the 1.2g of naphthalene. This agrees with the predicted trend because if there is
less of the substance within the solvent, the solute is more likely to freeze in its entirety. If there is a
generous amount of the solute in the solvent, there is a more likely hood that the solute would not
entirely freeze giving an inaccurate freezing point, and therefore and inaccurate molar mass value.
With this experiment there were a few sources of error because all of the data that was
recorded solely depended on the readings from the machines. For example, if the cooling machine
wasn’t at the desired temperature the substance may no freeze completely. If the cooling machine was
reporting a value that was below the freezing point, but it wasn’t actually at that temperature, the
solution may not completely freeze as desired giving an inaccurate recording of the freezing point. The
biggest source of error would be due to the thermistor reading. If there is a loose connection
somewhere throughout the apparatus, the resistance reading would be way off. If the resistance varied
that would then cause the calculated temperature to vary, providing an inaccurate freezing point. Also,
if the machine wasn’t calibrated accurately to the calibrated plot that we were given the values of the
freezing point would also be off. This calibration plot was used in order to determine the temperature of
the substance over time, using the values that were recorded for the resistance.
Garland, W.C.; Nibler, W.J.; Shoemaker,P.D. Experiments in Physical Chemistry,8; Hodge,L.H.; Thomas
Timp: New York, 2009; 179-187.