THE MOLECULAR WEIGHT OF A SOLUTE*

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1 Name Per THE MOLECULAR WEIGHT OF A SOLUTE* One of the most helpful things that can be known about an element or compound is its molecular weight (molecular mass, gram molecular mass, atomic mass, or gram atomic mass). Modern equipment exists that will break a compound apart and measure the mass of the resulting pieces thus obtaining the molecular weight directly. Since we do not have the money or space for such a machine, we must rely on more primitive methods for obtaining the molecular weight. The change in the freezing or boiling point of a solution is directly related to the molality of that solution. Knowing molality will allow us to calculate the molecular weight of the solute. In this experiment, our intention is to determine the molecular weight of an unknown compound by measuring the freezing point depression of a solution of the compound and cyclohexane. To perform the calculations there are four quantities that are necessary to measure: the mass of the cyclohexane the mass of the unknown compound the freezing point of the pure cyclohexane the freezing point of the solution. The mass of the cyclohexane can be determined by measuring the volume of the liquid used then calculating the mass using the density (density of cyclohexane = g/ml). The mass of the solute can be determined by weighting the sample on the analytical balance. Since solutions do not instantaneously freeze, the freezing point of the pure cyclohexane and the solution will have to be determined by measuring the temperature of the liquid as a function of time. Temperature Figure Time 60 A graph of some typical data is shown in figure 1. As the liquid (or solution) begins to cool the temperature begins to fall rapidly. However, as the liquid begins to freeze the temperature remains constant as long as the freezing is taking place. When the sample is completely frozen the temperature again begins to drop rapidly. The freezing point is the point at which the solvent begins to freeze. To obtain this value pick a range of points in the cooling portion of the graph that appears to be linear and find the equation of the line that best fits those values. Next find the range of points in the freezing portion of the graph that appears linear and also find the equation of that line. The freezing point is the y- coordinate of the intersection of the two lines (see figure 2). In the sample data shown the cooling portion of the data appears to be linear from points 1 to 5. The equation of the cooling portion is y = 2.62x The freezing portion seems to be linear from points 13 to 33, therefore the equation of that section is y = 0.014x The freezing point is the y-coordinate of the intersection of the two

2 Temperature Figure 2 lines or 7.5 C. When taking the data for the determination of a freezing point it is necessary to stir the sample continuously. The 25.0 cooling occurs on the outside of the test tube 20.0 and the thermometer is located in the center thus if stirring is not thorough the data taken is 15.0 not accurate I. Pre-lab Calculate the molality of a solution made by adding g of 0.0 benzophenone (C 13 H 10 O) to 10.0 ml of cyclohexane. The density of cyclohexane is g/ml. Time 2. Calculate the freezing point depression, ΔT f, for the solution in question 1. The freezing point depression constant for cyclohexane is 20.5 C/m. 60 II. Procedure Obtain an apparatus for determining the freezing point and a beaker of ice water. Make sure you record the number of the apparatus you are using since you want to use the same thermometer for the second lab period. First lab period 1. Pipette 25.0 ml of cyclohexane into the test tube part of the freezing point apparatus. Make sure to replace the stopper, thermometer, and stirrer quickly to prevent evaporation of the cyclohexane. 2. Place the apparatus in the ice bath and record the temperature every 15 seconds for ten minutes. Make sure you read the temperature to the nearest tenth (0.1) of a degree. Probably the easiest method is for one partner to stir and read the thermometer while the other partner observes the time and records the results. When the sample becomes difficult to stir, stop the stirring. Do not bend the copper stirrer. 3. After you have completed one set of measurements, allow the sample to melt and warm to room temperature. Repeat the experiment. 4. Pour the sample in the container in the hood. DO NOT wash out the apparatus with water! Second lab period 1. Weigh a piece of waxed paper on the analytical balance. 2. Use an electronic or triple beam balance to place between 0.30 and 0.32 grams of the unknown directly on the waxed paper. 3. Weigh the waxed paper and sample on the analytical balance. Don t forget to record the mass of the waxed paper and the waxed paper with sample on your data sheets. 4. Transfer the sample of unknown from the waxed paper into the test tube of the freezing point apparatus and add 25.0 ml of cyclohexane. Use a pipette to measure the cyclohexane. (Don t forget to replace the stopper quickly.) 5. Stir or swirl the solution until all the solute dissolves. 6. Determine the data for the freezing of the solution. Make sure you record your data. 7. After you have completed one set of measurements, allow the sample to melt and warm to room temperature. Repeat the experiment.

3 8. Pour the solution in the container in the hood. DO NOT pour it down the drain! III. Calculations 1. Use either graph paper or a graphing program on the computer to draw a graph of each set of data. Plot time on the X-axis and temperature on the Y-axis. Use the graphs to determine several points that represent the best straight lines for the cooling and freezing portions of the graph. 2. Use the program in Lab Check to get the equation of the best straight lines for the cooling and freezing portions of the graph. The program will figure the freezing point. Draw the lines on each graph. The lines should intersect at the freezing point. 3. Calculate ΔT f and the mass of solute for the experiment. 4. Calculate the molality of the solution. K f for cyclohexane is 20.5 C/m. 5. Calculate the molecular weight of the unknown. IV. Questions 1. Assume 12.5 g of the solute was dissolved in enough cyclohexane to produce 245 ml of solution. Use your calculated molecular weight to determine the molarity of the solution. 2. If the volume of cyclohexane used in this lab were doubled, what would be the molality of the solution formed?

4 V. Data Table: Cyclohexane CYCLOHEXANE Trial 1 CYCLOHEXANE Trial 2

5 VI. Data Table: Solution SOLUTION Trial 1 SOLUTION Trial 2

6 VII. Data Table: Summary Freezing point of the cyclohexane 1. Trial 1 C 2. Trial 2 C 3. *Average C Freezing point of the solution 4. Trial 1 C 5. Trial 2 C 6. *Average C 7. *ΔT f C 8. Mass of waxed paper g 9. Mass of paper and solute g 10. *Mass of solute g 11. *Mass of cyclohexane g 12. *Molecular weight of unknown g/mol