Determination of the Molar Mass of a Compound by Freezing Point Depression

Transcription

1 Determination of the Molar Mass of a Compound by Freezing Point Depression Objective: The objective of this experiment is to determine the molar mass of an unknown solute by measuring the freezing point depression of a solution of this solute in a solvent as compared to the freezing point of the pure solvent. Safety: Both the solvent and solute used in this experiment are flammable. Make sure the hot water bath used does not exceed 50 o C. Always remove the hot water bath from the heating plate before putting the test tube into the hot water bath. Always wear gloves when handling these substances This lab is a two week lab. In the first week you will measure the freezing point curve of the pure solvent. In the second week you will measure the freezing point curve of the solvent with a solute added to it. Introduction Colligative properties are properties of a solvent, such as freezing point depression and boiling point elevation, which depend on the concentration of solute particles dissolved in the solvent. The decrease in freezing point, Tf (freezing point depression), is defined as the difference in freezing point of a pure solvent vs. the freezing point of a solution composed of the solvent and a certain concentration of solute particles. The freezing point depression and the concentration of the solution are related by the equation: Tf = kf m (1) where kf is the freezing point depression constant of the solvent with units C kg solvent/mole solute. m is the molal concentration of the solute dissolved in the solvent expressed as moles of solute/kg solvent At the freezing point of any substance, an equilibrium exists in which both liquid and solid are present. A cooling curve can be used to determine the freezing point. To construct a cooling curve, one warms the sample, pure solvent or solution, to well above its melting point, then it is allowed to cool. As the sample cools, the temperature of the sample is monitored as a function of time. As the sample begins to solidify, the change in temperature will be slow, and in the case of a pure substance can be constant, until all the solution has solidified. A graph is made by plotting the temperature vs. time. An example of a cooling curve is shown below in Figure 1. In this cooling curve, you see a steady decrease in temperature followed by a dip which is followed by a slight rise in the temperature. This dip is not unusual and results from supercooling during the early stages of the freezing process. In this example, the dip is followed by a short plateau in the temperature. This plateau is at the freezing point of the pure solvent and corresponds to the time interval during which both solid and liquid are in equilibrium. When solute is added to the solvent, the shape of the cooling curve changes so that we don t see a clear

2 horizontal plateau as the example shown in Figure 1. Even in the case of a pure solvent we sometimes don t see a plateau. Following the solidification of the solution, the temperature drops more sharply as the solid begins to cool. The freezing point is determined from a cooling curve by drawing a line through the points in the region where the liquid is cooling and drawing a line through the points where the liquid is freezing. If both lines are extrapolated towards each other, their intersection gives the value of the freezing point. Figure 1. Cooling curves of a pure solvent and a solution made using the solvent In this week s experiment, you will determine the freezing point of pure tertiary butyl alcohol (tert-butanol, see figure 2) and the freezing point of tert-butanol with an unknown solute dissolved in it. From these freezing point measurements, you will be able to calculate the molar mass of the unknown solute. The tert-butanol is a good solvent choice for this experiment. Its transition state from solid to liquid occurs near room temperature, It also has a relatively large kf, 9.10 ºC kg solvent/mol solute, which is good for estimating the molar mass of a solute because it will allow us to see a large Tf following the introduction of a small amount of solute. Figure 2. Structural formula of tert-butanol, (CH 3) 3COH

3 Calculation of molecular weight The freezing point of the pure solvent and solution are determined from the cooling curves. The freezing point depression is calculated using Tf = freezing point pure solvent freezing point solution Equation 1, along with the value of kf (9.10 ºC kg solvent/mol solute), is then used to calculate the molality of the solution, m. The number of moles of solute present can be determined from the definition of molality moles solute = molality (mol / kg solvent) * mass solvent ( kg) The molar mass M of the solute is M = grams solute / moles solute

4 Prelab 1. The freezing point of a 50.0 g pure solvent is measured to be 45.2 o C. When 2.0 g of an unknown solute is added to the solvent the freezing point is measured to be 42.1 o C. If the freezing point depression constant of the pure solvent is 4.1 ºC kg solvent/mol solute what is the molecular weight of the solute? 2. The below graph shows part of a cooling curve in the region where the liquid is cooling and where the liquid is freezing. Two lines have been drawn through these regions. Estimate the freezing point of this liquid

5 3. Study the figure below. It shows a cooling curve for a solution containing an organic solvent and a solute. The full cooling curve taken over 600 s is shown in the inset of the figure. The main figure is a blow-up of the region (indicated by the dashed box) where liquid cooling becomes liquid freezing. In order to estimate the freezing point, it is necessary to draw two lines in this zoomed in region, one though the liquid cooling data points and one through the liquid freezing data points. The intersection of these lines is an estimate of the freezing point. Questions to ask yourself. Does this cooling curve have a supercooling region? How many points do we include when drawing the line though the liquid cooling region? Often science is somewhat more of an art than textbooks make it out to be. Each of the above questions does not have a correct answer. One tries one best and draw what you consider the best lines and then measures their intersection point 1. tempearture ( o C) temperature ( o C) 1 In more advanced settings, several lines are drawn and each is used to estimate the freezing point. The difference in these estimates is then used as a measure of the error in the value of the freezing point. A statistical technique known as propagation of errors can then be used to determine the associated error in the estimate of the molecular weight of the unknown.

6 Experimental Method The procedure for this experiment consists of melting and then freezing the solution of interest. This is done by placing the solution in a hot water bath, to melt it, and then an ice water bath, to freeze it. The cooling curve is measured while the solution is cooling/freezing in the ice water bath. Each pair of students will need two stirrers/hot plates. One of these will be used as a hot plate and the other as a stirrer The details of the procedure are given next. Week 1. Do steps 1 through 9 Week 2. Do steps 1 through 5 and then steps 10 through Make a hot water bath. Fill a 1000 ml beaker approximately half full with tap water. Place the beaker on a heating plate set to around 200 ⁰C. Allow the water to heat up to somewhere between ⁰C. You can check its temperature using a digital thermometer. 2. Obtain and weigh your test-tube containing your t-butanol. Obtain a test-tube containing the tert-butanol and a stir bar from your TA. Place your stir bar into the test tube. Place the testtube tube in a clean dry 250 ml beaker to support the test tube. Weigh this assembly on a tared mass balance and record its mass in table 1. Remember to always use the same balance for repeat weighing. Figure 3. Assembly used for weighing the solvent. Make sure you have placed the stir bar in to the test tube before weighing the assembly 3. Melt the tert-butanol. Once the temperature of the hot-water bath is optimal remove it from the hot plate. Place the test tube containing the tert-butanol into the hot water. Be careful to make sure that no water gets inside the test tube. Even a small amount will contaminate your sample. Gently swish the test tube around in the hot water bath and the tert-butanol will slowly melt to form a clear liquid 4. Setup the apparatus. Switch on the spark unit and make sure it is reading temperature correctly. Setup the apparatus as shown below using the spark unit s thermometer. Two test tube clamps will be required to hold the thermometer and the test tube in place. Adjust the height of the test tube so that the bottom of the test tube is close to but not touching the bottom of the beaker. Adjust the height of the thermometer so it tip is close to but not

7 touching the spin bar. Check the thermometer tip is positioned centrally relative to the sides of the test tube. It should not be touching the sides of the test tube. The level of the liquid in the water bath should be above the level of the tert-butanol in the test tube. (Later in the experiment you will be required to replace the water bath. To do this you can simply lift the stand/thermometer/test tube assembly by lifting the stand. This will save you having to realign the thermometer and test tube each time you need to change the water bath). Switch on the stirrer to its maximum stir rate. Figure 4. Apparatus setup. Note that the stirrer is not placed on the base of the stand. 5. Construct an ice bath. Prepare an ice bath by filling a 1 L beaker with approximately 800 ml of ice. Add cold tap water until the ice is just covered with water. Ensure the bath is mixed by stirring the bath gently with a glass rod stirrer. 6. Measure the cooling curve of the solvent/solution. Remove the test tube and thermometer assembly from the hot water bath by lifting-up the stand. Remove the hot water bath from the top of the stirrer. Place the ice bath on top of the stirrer and gently lower the test tube stand assembly into the ice bath. Immediately begin to take temperature readings and record them in table 3 every 15 seconds. Continue taking readings for 8 minutes. 7. Re-melt the solvent. Check the hot water bath is in the optimal temperature range. If it isn t place it back on the hot plate until it reaches the optimal range. Remove the hot water bath from

8 the hot plate. Using the stand, lift the test tube/thermometer out of the ice bath. Swap the ice bath for the hot water bath on top of the stirrer and then lower the test tube/thermometer into the hot water bath. After some minutes, the tert-butanol will melt and the stir bar will resume stirring. 8. Plot a cooling curve with your data. Does it look like figure 1? If not, make sure that the tip of the thermometer is positioned centrally in the solvent, not touching the sides. Remeasure the cooling curve of the solvent repeating step 6 and step Remeasure the cooling curve of the solvent. Repeat step 6 and step 7. The following steps will be carried out during the second week of the lab 10. Add the solute to the tert-butanol and reweigh. Before adding the solute, you will need to re-melt the solvent (step 7). Once this is done, lift the test tube out of the hot water bath. Detach the test tube from the clamp and dry its outside. Take it to the fume hood and add approximately 1 ml of the unknown to the test tube. Gently swish to mix and then reweigh the test tube containing the solvent, stir bar and the unknown solute as in step Reconstruct the apparatus. Repeat step Measure the cooling curve of the solution. Repeat step Remeasure the cooling curve of the solution. Repeat step 7 to re-melt the solvent and then repeat step Remeasure the cooling curve of the solution. Repeat step 7 to re-melt the solvent and then repeat step Disposal. To dispose of the solution, you will need to re-melt the solution as in step 7. Once it has melted, empty the contents of the test tube into the appropriate waste container. Be careful not to drop the stir bar into the waste container. Wash the thermometer with water and then acetone. Wash the test tube and stir bar with water and then acetone. Once they are both dry, place them both in the 250 ml beaker and weigh them. Enter the information in table 1. Things to note 1. Depending on how long the experiment takes you (and the temperature in the lab), you may need to refresh your ice bath during the experiment by pouring off some water and adding a little more ice to it. 2. Pay careful attention not to splash any water into the tert-butanol when transferring it between and hot and cold water baths. 3. Always remove the hot water bath from the hot plate before placing the test tube into it.

9 Data analysis (week 1). 1. Plot three graphs on the same piece of graph paper, one for each cooling curve. Label axis and title the graph. 2. For each graph, In the vicinity of the freezing point, draw a line through the points where the liquid is cooling and a line through the points where the liquid is freezing. 3. Enter the three respective freezing points into table 3 Data analysis (week 2) 1. Plot three graphs on the same piece of graph paper, one for each cooling curve. Label axis and title the graph. 2. For each graph, In the vicinity of the freezing point, draw a line through the points where the liquid is cooling and a line through the points where the liquid is freezing 3. Compete the rest of table 3 4. Use your data to calculate the molecular weight of your unknown. Please show your working

10 Data Report Sheet 1 Name room# desk # Table 1a (week 1). Weight measurements mass of test tube + beaker + stir bar + solvent = mass of test tube + beaker + stir bar = mass solvent used = Table 1b (week 2). Weight measurements mass of test tube + beaker + stir bar + solvent = mass of test tube + beaker + stir bar + solvent + solute = mass of test tube + beaker + stir bar = mass solvent used = mass solute used = Table 3. Freezing point depression determination Freezing point trial 1 trial 2 trial 3 average Pure solvent (week 1) Solvent +solute (week 2) Tf = Calculation of molecular weight (use data from table 1b and table 3) Please show your working Molecular weight = Please hand-in DRS one through four TA signature

11 Data Report Sheet 2 Name room# desk # Table 2. Cooling Curve Data Week 1 Week 2 time (min:s) T (pure solvent) T (pure solvent) T (pure solvent) time (min:s) T (solution) T (solution) 0:0 0:0 0:15 0:15 0:30 0:30 0:45 0:45 1:00 1:00 1:15 1:15 1:30 1:30 1:45 1:45 2:00 2:00 2:15 2:15 2:30 2:30 2:45 2:45 3:00 3:00 3:15 3:15 3:30 3:30 3:45 3:45 4:00 4:00 4:15 4:15 4:30 4:30 4:45 4:45 5:00 5:00 5:15 5:15 5:30 5:30 5:45 5:45 6:00 6:00 6:15 6:15 6:30 6:30 6:45 6:45 7:00 7:00 7:15 7:15 7:30 7:30 7:45 7:45 8:00 8:00 T (solution)

12 Data Report Sheet 3 Name room# desk #

13 Data Report Sheet 4 Name room# desk #