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Data Sheet 1 L. Determining the Freezing Pointer Water determination determinat 1 top,c 2 1 2 temp. "C timin mem Im "c temp. : Determining Molar Mass by Freezing Point Depression 7.5 0:0 دک 80 0.0 0.5 1.0 15 0.2 12 0.6 : -2.44 -1.5 8.5 9.0 9.5 10.0 10.5 20 0:0 0.0 -0.4 2.5 3.0 3.5 11.0 11.5 -0.9 -1.8 0.0 12.0 4.0 4.5 5.0 5.5 12.5 0.0 13.0 6.0 13.5 14.0 6.5 7.0 14.5 Data Sheet 2 II. Determining the Freezing Point of an Unknown Solution unknown identification code determination Graduated cylinder 1 2 mass of die water, and unknown, & GC nass of de and unknown, & woter Bass of 8 mass of Salt 9 130.695 114.259 16.436 0.027 11
determination determination 2 tomt. "C temps, 1 2 tетр, °C time in time in temp., °C 00 5:0 75 -4.0 0.5 1.0 1.3 8.0 8.5 Chapter 1: Determining Molar Mass by Freezing Point Depression 1.5 9.0 2.0 -1.2 -269 -2.6 -2.4 -- 2.8 9.5 10.0 2.5 3.0 10.5 3.5 11.0 4.0 11.5 - 3.5 -4.4 -50 -5.4 -5.6 - 5.8 - 6.0 -6.0 -6.3 -6.3 -6.4 -6.6 4.5 120 12.5 5.0 5.5 13.0 -3.4 -2.5 -3.6 -3.8 -3.9 - 4.0 6.0 13.5 6.5 14.0 2.0 14,5 determination 2 2.997 9.44 00944 O degree Celcius mass of unknown, 8 mass of water, g mass of water, kg freezing point of water, °C average freezing point of water, °C freezing point of unknown solution, °C freezing point depression for solution, At,, °C molality of unknown solution, mol/kg molar mass of unknown, g/mol average molar mass of unknown, g/mol
Pre-Laboratory Assignment 1. Explain why it is necessary to use an ice-salt water bath, rather than an ice-water bath, to cool the water solutions in this experiment, molecules inuid State ale under constant motion due to operater untia energy, moreover, the heat absorbed Wato, molecules Vand state is dissipated more acecently than the soud state. These com ice cold Guidweek atocis a better color than sido ide at oc 2. Using the Procedure of this experiment, a student determined the molar mass of an unknown non- electrolyte compound by measuring the freezing point depression of an aqueous solution of the compound. The student obtained the following data: mass of empty test tube, 8 35.223 mass of test tube and compound, g 36,837 mass of test tube, compound, and distilled water, 56.984 freezing point of distilled water, * Per 1: Determining Mass by Freezing Point Depression 0.05 time, min temperature, C time, in temperature, 0.0 0.5 1.0 1.5 2.0 25 3.0 3.5 4.0 4.5 2.25 1.57 1.00 0.46 -0.07 -0.56 -1.02 -1.42 -1.78 -2.12 DONNOO GOO 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 -2.43 -2.72 -2.97 -1.71 -1.73 -1.75 -1.78 -1.80 -1.83 UU (a) Plot the temperature-time data, and use it to determine the freezing point, tt, for the solution (b) Calculate the freezing point depression, At, for the solution. (c) Calculate the molality of the solution, m, in moles of solute per kilogram of water. (d) Calculate the molar mass, MM, of the unknown.
3. The temperature-time data in Assignment 2 indicates that, after the unknown solution super-coole to -2.97 °C its temperature suddenly rose to - 1.71 °C, and then gradually drifted downward again. In order to determine the true freezing point of the solution, you extended the horizontal portion of the curve until it intersected the first part of the curve (see Figure 2). The intersection corresponded to the true freezing point of the solution. If you used -1.71°C as the freezing point of the solution, Instead of -1.61 °C how would it affect your calculated molar mass for the unknown? Chapter 1: Determining Molar Mass by Freezing Point Depression 4. Cyclohexane (CH2) can be used as a solvent for freezing point depression measurements. When 1.07 g of naphthalene (CH) (MM = 128.17 g/mol) is mixed with 51.29 g of cyclohexane, the freezing point of the resulting solution is 3.29°C. The freezing point of pure cyclohexane is 6.54°C. (a) Calculate the freezing point depression of the solution (b) Calculate the molality of the solution (e) Calculate the molal freezing point depression constant for cyclohexane. (d) It is known that in a nonpolar solvent such as cyclohexane, trichloroacetic acid (CCI,COOH) exists as hydrogen-bonded dimers, with the following structure: OH-0 CI CK CH -C-C CI CI O-H----O CI Suppose we prepared a 0.20 molal solution of trichloroacetic acid in cyclohexane. What would you expect the freezing point depression of this solution to be? Explain your answer.