Gonzalez, Margarita G. Date of Experiment: February 02, 2015Gutierrez, Jillian M. Date Submitted: February 27, 2015

EXPERIMENT # 3Determination of the Molecular Weight of a Non-Volatile Solid

by Cryoscopic Method

I. Introduction

The properties of a pure solvent differ from that of a solution due to the interactions taking place between their molecules. Colligative properties are the physical changes arising from the addition of solute to a solvent. These properties include vapor pressure lowering, boiling point elevation, freezing point depression and changes in osmotic pressure. These properties are dependent only on the number of particles that are dissolved in the solvent and not on their identity.

In this experiment, the goal is to determine the weight of an unknown solute using the freezing point depression method or the cryoscopic method. The observed freezing point depression and the measured weight of the solute, which is benzoic acid, can be used to determine the molecular weight of an unknown solute. The solvent used in this experiment is glacial acetic acid.

II. Methodology

The experiment was performed in the fume hood and 3 trials were made per set of procedure.

For the determination of the cryoscopic constant of the solvent, which is the glacial acetic acid, 15 mL of glacial acetic acid was measured then transferred into the hard-glass test tube. This test tube containing the acid was dipped in an ice water mixture and the temperature was taken at 10-second intervals until successive readings are the same. The test tube was then transferred into an empty beaker until it reaches room temperature. These steps were repeated twice and the results were recorded. Using the same acetic acid used in the first trial, 1 gram of benzoic acid was completely dissolved. Again, the test tube was dipped in the ice water mixture while the temperature was taken at 10-second intervals. This was repeated twice, using varied masses of benzoic acid between 1.0-1.5 g and different volumes of glacial acetic acid.

For the determination of the molecular weight of an unknown sample, these steps repeated but instead of using benzoic acid, an unknown solute provided by the instructor was used.

III. Results and Discussion

Figure 1 Temperature vs time graph of pure glacial acetic acid and acetic acid-benzoic acid mixture

FREEZING POINT, ˚CPure Glacial Acetic Acid 14.1315mL GAA with 1.019g benzoic acid 13.220mL GAA with 1.0193g benzoic acid 11.825mL GAA with 1.2998g benzoic acid 10.8

Table 1 Freezing point readings of pure acetic acid and acetic acid-benzoic acid solutions

From the given figure and table above, the freezing point of a certain substance can be determined by reading the highest point on the graph, which is the freezing point, after the lowest point, which the substance starts to crystallize indicating that freezing is nearly to occur.

Based on the results of the experiment, the freezing point of pure acetic acid is lowered when a certain amount of solute is dissolved in it. This occurrence is called the freezing-point depression in which the addition of a solute to a solvent decreases the freezing point of the solvent. The freezing point of pure glacial acetic acid is higher than the freezing point of the impure acetic acid solution.

The cryoscopic constant, Kf, of glacial acetic acid was determined by using these equations:

Kf = ΔTf xMW of solute x massof solvent

massof solute

ΔTf = Tpure solvent – Tsolution m= ρv

Wherein benzoic acid is the solute while acetic acid is the solvent.

TRIAL Kf, ˚C kg/ mole1 15mL GAA with 1.019g benzoic acid 1.740862 20mL GAA with 1.0193g benzoic acid 5.813623 25mL GAA with 1.2998g benzoic acid 8.14461

Average Kf 5.23Table 2 Computed Cryoscopic constants for each trial

Comparing the calculated average Kf from the experiment versus the theoretical Kf

which is 3.90 ˚C kg/ mole gave a percentage error of 34.10%.

The molecular weight of the unknown sample can be determined by using this equation:

MWx = Kf xmass of soluteΔTf x massof solvent

TRIAL MWx, g/mol1 15mL GAA with 1.021g of X 78.952 20mL GAA with 1.0185g of X 192.313 25mL GAA with 1.2979g of X 111.91

Average MW 127.72Table 3 Computed molecular weights for each trial

Comparing the calculated average MW from the experiment versus the theoretical MW which is 128.1632 g/mol gave a percentage error of 0.35%.

The amount of acetic acid used did not affect the calculated molecular weight of the unknown solute. So if the amount of glacial acetic acid used is more than 15 ml of less than 15 ml there would be no difference in the calculated molecular weight. Instead, it is the freezing point that was affected by the amount of the acetic acid.

Without looking at the result of the experiment, we first thought of the sample to be naphthalene because of its strong odor that smells like mothballs. Secondly, because of its physical property which is white/colorless crystalline solid.

Supercooling is to cool a liquid below its freezing point without producing solidification or crystallization. This phenomenon occurred in all parts of the experiment. Freezing point is not affected; however, its observation may be.

IV. References

1. http://www.bc.edu/schools/cas/chemistry/undergrad/gen/spring/FPDep.pdf

2. http://dictionary.reference.com/browse/supercooling