Pre-Lab: Example Problem: Determine the molecular weight of acetic acid if a solution that contains 30.0 grams of acetic

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Pre Lab Example Problem Determine The Molecular Weight Of Acetic Acid If A Solution That Contains 30 0 Grams Of Acetic 1 (179.67 KiB) Viewed 26 times

Pre-Lab: Example Problem: Determine the molecular weight of acetic acid if a solution that contains 30.0 grams of acetic acid per kilogram of water freezes at - 0.93°C (you know the freezing point of pure water). Do these results agree with the assumption that acetic acid has the formula CH3COOH? Solution The freezing point depression for this solution is equal to the difference between the freezing point of the solution (- 0.93°C) and the freezing point of pure water (0°C). Tf = 0.93°C Using the equation that defines freezing point depression, the molality can be determined: A Tf = Kfx m Since we know the change in the freezing point, and K, is a constant (1.86 °C/m for water): m = ΔΤf Kf 30.0g 0.50mol 0.93 °C 1.86 °C 372 At this point, return to the statement of the problem, to see if you are making progress toward an answer. According to this calculation, there are 0.50 moles of acetic acid per kilogram of water in this solution. The problem stated that there were 30.0 grams of acetic acid per kilogram of water in the solution. Since we simultaneously know the number of grams and the number of moles of acetic acid in this sample, we can calculate the molecular weight of acetic acid. 60, 9 mol = 0.50 m The results of this experiment are in good agreement with the molecular weight (60.05 g/mol) expected if the formula for acetic acid is CH3COOH. Now it is your turn: A solution of 2.41 grams of solute in 77.0 grams of a solvent (Kf = 8.44 for the solvent; Tfreezing of the pure solvent = 81.9 °C) is found to have a freezing point of 78.6 °C. What is the solute's molecular weight? (Hint: There is one difference in this problem compared to the example....the solute is dissolved in only 77.0 grams of solvent. You will need to add one step to obtain the correct answer.) _g/mol Your answer should have 3 sig figs.