Experiment 13: Determination of Molecular Weight by Freezing Point Depression

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1 1 Experiment 13: Determination of Molecular Weight by Freezing Point Depression Objective: In this experiment, you will determine the molecular weight of a compound by measuring the freezing point of a solution of the compound and then comparing the freezing point of that solution to that of the pure solvent. Introduction The properties of a solution differ from those of a pure solvent due to interactions that take place between the solute and solvent molecules. The properties that exhibit such changes are called the colligative properties and include vapor pressure lowering, boiling point elevation, freezing point depression and changes in osmotic pressure (see Tro, section pp ). These properties are dependent only upon the number of particles (ions or molecules) that are dissolved in the solvent and not on the identity of the particles. This experiment will examine the phenomenon of freezing point depression. true: When a particular solute is dissolved in a solvent, the following expression holds ΔT = T f T f = K f m The terms T f and T f refer to the freezing-point temperatures of the pure solvent and the solution, respectively. The term m indicates the molality of the solution, which is defined as the number of moles of solute per 1000 g of solvent. This quantity is used, rather than molarity, because it is not temperature dependent. The constant, K f, is referred to as the freezing-point-depression constant and is dependent only upon the solvent. The change in temperature is also dependent upon the number of solute particles in solution the more particles that are present, the larger the change in temperature. For this reason, the above equation is sometimes written as: T f - T f = K f im, where i = the number of solute particles produced per formula unit that dissolves. In a solution containing an electrolyte, each ion is considered to be a particle. You will explore the influence of i on the freezing point depression in Part II of the experiment. In Part I of the experiment, you will use cyclohexane, an organic compound that is a liquid at room temperature, as the solvent. You will be able to determine the molecular weight of an unknown compound by observing the freezing point of a solution of the compound in cyclohexane and comparing it to the freezing point of pure cyclohexane. Calculations 1. The compound cyclohexane has a melting point (or freezing point) of about 6 C. You will obtain a series of temperatures of pure cyclohexane as it cools down from room temperature through its freezing point in an ice bath. These temperatures will be plotted as a function of time. Similarly, you will obtain temperatures of a solution of an unknown compound in cyclohexane as it cools down to the freezing point, which will also be plotted. The plots will look similar to those pictured in Figures 1a and 1b, and the T f and T f values can be extrapolated from them as shown on Figure Page 1, Expt. 13.

2 2 2. The molality of a solution can be expressed in terms of the molar mass of the solute: m = moles of solute 1 kg of solvent = (g of solute/molar mass solute,g/mol) (g of solvent) (kg/1000 g) = (1000 g/kg) (g of solute) (molar mass solute, g/mol) (g of solvent) Substituting this expression into the equation for freezing-point depression we obtain: o ΔT=T f K ) (g of solute) (molar mass solute,g/mol) (g of solvent) T = (1000 g/kg) ( f f Eq. 1 For cyclohexane, K f = 20.2 deg kg/mole of solute. Recall that the molecular weight (in amu) of a substance has the same numerical value as its molar mass. In Part II of the experiment, you will measure the change in temperature when urea, CON 2 H 4, is added to a mixture of ice and water and compare it with the change in temperature observed when sodium chloride, NaCl, is added to an ice-water mixture.

3 3 Procedure (you do not need to outline instructions for using the LoggerPro software) A temperature probe interfaced to a computer will be used to acquire the temperature readings in this experiment. Two students will use the same computer, but each student will have his/her own temperature probe and will obtain his/her own set of data to evaluate. Two temperature probes should already be plugged into the LabQuest 2 interface. Choose a temperature probe and record the number of the channel into which it is plugged. You must use the same probe for all measurements. The temperature probes have an uncertainty of ±0.1 C. To log onto the computer, type student for the name and type chemistry for the password, then click on Log In. Click on the Applications folder at the bottom of the screen. In the window that opens, click on the button labeled Logger Pro to open the Logger Pro software. When two temperature probes are plugged into the LabQuest 2 interface, the following picture should appear on the screen: Pull down the Experiment menu and choose Data Collection. Change the length of the experiment to 800 seconds. The Sampling Rate should be set to 1 samples/second. Click on Done. Click once on the number at the very top of the y- axis. Type in 40 and press return. You are now ready to collect data. Wear safety glasses at all times. The computer will collect the data for both your and your partner's temperature probes simultaneously; therefore, you should carry out the steps of the experiment at approximately the same time. Part I. Determining the Molecular Weight of an Unknown Compound Wipe the temperature probe with a paper towel to be sure it is dry. Assemble your apparatus as in Figure 2, shown on Figure Page 2, Expt. 13. A 16x150 mm test tube will be provided for you. The test tube must be clean and dry! Insert the stopper with

7 7 2. In the window that appears, scroll through the list, clicking on the boxes until the box(es) corresponding only to the data that you wish to display has/have a check mark. When you are ready to leave, pull down the Logger Pro menu and choose Quit Logger Pro. You should leave the laboratory with printouts of all the necessary plots that you produced today. However, you should also store a copy of your data file(s) using either or by ing the file(s) to yourself (see Storing and Retrieving Your Lab Files on page 14 of the notebook/manual). If you need to adjust any of your plots after your lab period is over, you can use the computers in the chemistry teaching labs when the rooms are not occupied (M, W at 9:00 1:00 and Tu, Th at 12:00 1:15).

8 8 Calculations 1. Determine T f for the pure cyclohexane and T f for the unknown solution from the printouts of your plots as shown in Figures 1a and 1b on Figure Page 1, Expt Calculate the molecular weight (in amu) of your unknown using Eq What was your code number? Identify your unknown compound from the following list and explain your conclusion. Biphenyl; (C 6 H 5 ) 2 4-Bromochlorobenzene; Br(C 6 H 4 )Cl Naphthalene; C 10 H 8 Calculate your percent error for the molecular weight of your unknown: % error = accepted value experimental value x 100 accepted value 4. a) Calculate the molalities of the urea and sodium chloride solutions. To do this, use the mass of slush (ice plus water) and the mass of solute (urea or sodium chloride) that you measured. b) Determine the ΔT s for the urea and sodium chloride solutions, using the lowest portion of the plots to determine T f (see Figure 3 on Figure Page 2, Expt. 13). Questions 1. Why is molality not temperature dependent while molarity is temperature dependent? 2. A student s unknown compound contained an impurity that is insoluble in cyclohexane. What is the effect on the molecular weight determination? 3. a) Thoroughly discuss why the values of ΔT for the urea and sodium chloride solutions are different. b) A spike in temperature is often seen in the plots from Part II when the solid is first added to the water (Figure 3 on Figure Page 2, Expt. 13). Provide a possible explanation for this initial increase in temperature (see Chapter 12 in Tro), and then describe an experiment that would allow you to test your theory. 4. List possible sources of error in Part I of this experiment and for each, indicate whether the effect would be an erroneously high or low value for the molecular weight of the unknown.

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