Freezing Point Depression Experiment

Discussion: In this experiment, freezing point depression was used to determine the molar mass of three unknown solids. First, the freezing point of the mixture of ice and water was determined, as freezing occurs when both the solid and liquid phases pre-present together. The temperature when the ice and water mixture reached equilibrium was recorded. Then, the solution of the unknown solids was prepared and its freezing point was determined. Then the equation of molarity allowed the experimenters to determine the number of moles present and as a result, the molar mass also, identifying the three solids. By excluding the outliers and taking an average of the molar masses determined from the class data, it can be concluded that unknown A was

For example, for unknown A, by taking an average of the class data, excluding the outlier of 377.13 g//mol, it can be concluded that the average molar mass is about 63 g/mol, which is closest to potassium chloride with a molar mas of 74.551 g/mol. Moreover, by excluding the outlier of 934.57 g/mol from another group and the personal molar mass determined by this personal experiment of 758.38 g/mol, the average molar mass is about 260 g/mol, which is closest to table sugar with a molar mass of 342.297 g/mol. Similarly for unknown C, excluding the outlier of 377.13 g/mol, the average molar mass is about 60 g/mol, which is closest to sodium chloride with a molar mass of 58.443 g/mol. Nevertheless, the molar masses determined from the personal experiment was lower than what was expected, with molar masses of 39.069 g/mol for unknown A, 758.38 g/mol for unknown B, and 42.26 g/mol for unknown C. Therefore, there was definitely error. The ionic solids potassium chloride and sodium chloride split into two particles in solution, so their concentration should be doubled, which needs to be accounted for more accurate

This method is nearly always incorrect because it is difficult to obtain a perfectly saturated solution. In most cases, the solution remains unsaturated or it does not dissolve all the way. Also, for measuring the molar mass using freezing point depression for ionic compounds, it is important to determine the concentration of the particles of the solute, as the Van Hoff factor is not one, as it is for molecular solids. According to Atkins Peter, author and chemist of the book, Physical Chemistry says, “In predicting the expected freezing point of a solution, one must consider not only the number of formula units present, but also the number of ions that result from each formula unit, in the case of ionic compounds.” Therefore, this method works, but it is approximate and it works better for low solute concentrations. The presence of solute lowers the freezing point, which in turn affects the calculations to determine the molar mass of the unknown. Henceforth, there are discrepancies to determine the identities of these