The effect that solutes have on the freezing point of a solvent and to determine the molar mass of a solute

The purpose of the experiment was to observe and measure the effect that solutes have on the freezing point of a solvent and to determine the molar mass of a solute. Theories examined during the conduction of the experiment included solutions, colligative properties, vapor pressure, freezing point, boiling point, molality and molar mass. Techniques applied during experimentation included ice/water bath preparation, determination of freezing point, determination of molality, determination of molar mass, and data plotting. The results obtained from experimentation were: the average molar mass of the unknown solute was 177.23g/mol with a standard deviation of 30.5 and a relative standard deviation of 17.21%. The hypothesis was accepted.

The purpose of the experiment was to observe and measure the effect that solutes have on the freezing point of a solvent and to determine the molar mass of a solute. Concepts explored throughout the conduction of the experiment included solutions, colligative properties, vapor pressure, freezing point, boiling point, molality and molar mass.

A solution could be defined as a homogenous mixture formed by the addition of a soluble solute to a liquid solvent (Beran, 2009). Solutions could be gasses, liquids or solids. From experimentation it was observed, that the solvent’s physical properties may experience some change due to the amount of solute added to the solution. The physical properties that depend on the amount of solute added and not on the chemical properties of the solute were called colligative properties; such properties include: freezing point depression, boiling point elevation, vapor pressure decrease and osmotic pressure increase (Zumdahl, 2009). During the experiment, it was seen how the freezing point of a solvent decreased as the amount of solute added increased.

The freezing point of a solution was described as the point where the liquid a solid phases coexist (Beran, 2009). The boiling point of a liquid happens at the temperature where the vapor pressure equals atmospheric pressure (Zumdahl, 2009). Vapor pressure is the pressure applied during the state of dynamic equilibrium between a vapor and its liquid; vapor pressure decreases as the amount of solute added to the solution increases (Beran, 2009). Molality is represented by moles of solute/kg of solvent; molality was used to determine the moles of solute which in turn aided in the calculation of the molar mass. Molar mass or molecular weight refers to the mass of one mole of molecules (Zumdahl, 2009).

Techniques used in the experiment included: Preparation of an ice/water bath, preparation of cyclohexane solvent, preparation of solute sample, recording and plotting of data, and

calculation and determination of results. The hypothesis stated that the molar mass of a solute can be determined by the calculation of the freezing point depression of a solution; the hypothesis was accepted since the molar mass of the unknown solute was successfully determined to be 177.23g/mol.

Materials/Methods

For materials and methods used in the above mentioned experiment refer to the Laboratory Manual for Principles of General Chemistry (pages.183-188). There were no deviations from the original procedure.

Discussion

The molar mass of an unknown solute B was calculated to be 117.23g/mol by experimentation this was accomplished by the determination of the freezing point and the molality of a solution. For the experiment, cyclohexane was used as a solvent, the solute remained unknown.

During the first part of experimentation, an ice/water bath was prepared in order to determine the freezing point of pure liquid cyclohexane; the time it took for the cyclohexane to freeze was recorded in intervals of 30sec, this was done to be able to later compare the freezing rate of pure cyclohexane to the freezing rate of the solution of cyclohexane plus the solute. It was determined through experimentation that the freezing point of pure cyclohexane was 70C.

Then 0.198g of an unknown solute was added and the same process repeated for the determination of the freezing point of the newly formed solution. The freezing point for the first addition of solute was determined to be 4.5oC. More solute was added in order to see how the amount of solute added to a solvent affected its freezing point. By the last trial performed, 0.570g of solute was added to the cyclohexane and the freezing point of the solution was determined to be 1.8oC. The fact that the freezing point of the solution kept decreasing as more solute was added supports the theory that colligative properties are affected by the amount of solute particles present and not by the chemical properties of the solute (Beran, 2009).

Because T= Kf x m, the molality of the solution was calculated, molality aided in the calculation of moles of solute which in turn helped in the determination of molar mass.

From the experiment, the importance of solutions and colligative properties were obvious; solutions are all around us, the significance of understanding how substances interact with each other helps in the realization of how life really works.

Conclusion

The results obtained from the experiment were as follows, the average molar mass of the unknown solute was 177.23g/mol with a standard deviation of 30.5 and a relative standard deviation of 17.21%. The hypothesis stated that the molar mass of a solute can be determined by the calculation of the freezing point depression of a solution; the hypothesis was accepted since the molar mass of the unknown solute was successfully determined by the study of certain colligative properties such as freezing point depression.

Experimental errors that could have affected the results of the experiment included: inconsistent recording techniques of the freezing point of the solution and the use of different balances for acquiring measurements throughout the experiment. The two errors could cause a variation in the results. Improvements in experimentation that could help in obtaining more accurate results include: Proper techniques and procedures applied when determining the freezing point of the cyclohexane during the whole experimentation process and using the same tared balance all the way through the experiment since different (inaccurate) results may be obtained if different balances were used.