Unformatted text preview: CSUS Department of Chemistry Experiment 10 Chem. 1A EXPERIMENT 10: PRE‐LABORATORY Name: ______________________________ ASSIGNMENT Section: __________ Score: ______ / 10 1. Ideally, colligative properties depend only on the? 2. If 86.5 g of ethanol (C2H5OH) is added to 355 g of water, what is the molality of the ethanol solution? Ans. __________________ 3. Which of the following aqueous solutions should have the lowest freezing point? a. pure H2O b. 1 m MgBr2 c. 1 m RbI d. 1 m NH3 e. 1 m CH3OH Why? 4. What is the freezing point of a solution containing 3.10 grams benzene (molar mass = 78.11 g/mol) dissolved in 32.0 grams paradichlorobenzene? The freezing point of pure paradichlorobenzene is 53.0 °C and the freezing point depression constant, Kfp, is –7.10 °C/m. Ans. __________________ 5. What is the molar mass of an unknown molecular compound if 3.42 grams dissolved in 41.8 grams benzene begins to freeze at 1.17 °C? The freezing point of pure benzene is 5.50 °C and the freezing point depression constant, Kfp, is –5.12 °C/m. Ans. __________________ Page 1 of 7 CSUS Department of Chemistry Experiment 10 Chem. 1A EXPERIMENT 10: MOLECULAR WEIGHT DETERMINATION FROM FREEZING POINT DEPRESSION Introduction Some properties of solutions differ from the properties of the pure solvent. For example, we when a solute is dissolved in water to make an aqueous solution, we observe that the freezing point of the solution is lowered compared to the 0 °C freezing point associated with pure water. Similarly, the boiling point of the solution increases above the expected 100 °C associated with pure water. The observed freezing point depression and boiling point elevation depends on the quantity (concentration) of solute particles and not the kind of solute particles present. Freezing point depression and boiling point elevation represent two of a group of solution properties called colligative properties. Other colligative properties include osmotic pressure and vapor pressure reduction. In this laboratory experiment, you will utilize the observed freezing point depression of a solution to determine the molecular weight of an unknown solute. When only a small amount of solid solute is added to a solvent, the amount of change in either the boiling or freezing point is directly proportional to the concentration of the added solute. Expressing the solute concentration in molality, m = moles solute kg solvent (1) the freezing point depression of the solvent follows the relationship ΔTf = Kf × m (2) where ΔTf is the freezing point depression, ΔTf = Tf (solution) −Tf (pure solvent), and Kf is the cryoscopic constant. The cryoscopic constant is a constant associated with a particular solvent and can be looked up in a reference book. For example, Kf = −1.86 °C kg/mol for water, −4.90 °C kg/mol for benzene, and −9.80 °C kg/mol for benzophenone. Equation (2) can be rearranged as follows: m = ΔTf Kf (3) Then, using equation (1), moles of solute = m × kg solvent (4) and the molecular weight (MW) of the solute can be determined by substituting the moles found in equation (4) into Molar mass = mass of solute moles of solute (5) In this experiment you will measure the freezing point depression caused by a known mass of an unknown solute dissolved in a known mass of benzophenone (BZP) solvent. You will then use equations (3)‐(5) to calculate the molecular weight of your unknown solute. Page 2 of 7 CSUS Department of Chemistry Method Experiment 10 Chem. 1A A convenient method for measuring freezing points involves the use of “cooling curves.” The upper curve in the figure below shows what happens when pure BZP is heated above its melting point and is then allowed to cool in air. The liquid cools fairly rapidly until the temperature at which solid begins to freeze out. The temperature will then remain constant at the freezing point of BZP for several minutes, as more and more solid freezes. The lower curve in the figure shows what happens when a BZP solution (with an added solute) is cooled. The liquid cools to a temperature below the freezing point of pure BZP (notice the curve is lower than the pure BZP curve). When solid BZP begins to freeze out, the rate of cooling slows dramatically, but the temperature does not remain constant. The best way to determine the temperature at which solid first starts to appear is to draw a straight line through several points before the break in slope and another line through several points afterwards. The intersection of the two straight lines is the freezing point of the solution. (Sometimes a liquid may supercool slightly before the first crystals of solid appear. Note that the supercooled points are not used in determining the freezing point.) Your lab instructor will show you how to determine the freezing points from your graphs. Page 3 of 7 CSUS Department of Chemistry Experiment 10 Experimental (Work in pairs for this experiment) Chem. 1A A. Pure Benzophenone (BZP) Obtain a stopper fitted with a digital thermometer and stirrer from your T.A. Weigh a clean, dry 25 x 200 mm (long, wide) test tube on the analytical balance to the nearest 0.0001 g. Add about 10 g of BZP to the test tube and weigh again. Record the mass. Fill a 600 mL beaker almost full with hot water from the tap and place it on an iron ring and wire gauze above a Bunsen burner. Clamp the test tube to the ring stand and immerse it in the water so that all the BZP is below the water level. Place the thermometer from your locker in the water bath and heat with the burner until the water temperature is between 55 and 60°C. At this point the BZP should be at least partially melted. Turn off the burner and clamp the thermometer‐stirrer assembly above the test tube so that the bulb of the thermometer is about 1 cm above the bottom of the test tube. Stir until all solid has melted. Now remove the test tube from the water bath and dry the outside of the test tube thoroughly. Stir the liquid slowly and continuously, and observe the temperature. Start recording temperature readings at about 55 °C, taking a reading every 30 seconds. Stop when the temperature has bounced back from super‐cooling and remained about constant for five or six readings. B. BZP plus unknown solute Weigh your unknown and its container to 0.0001 g. Pour about half the sample onto a weigh paper and record the weight. Addthe weighed portion of your unknown into your test tube containing the BZP. Then proceed as in part A, making certain that all BZP on the test tube walls is melted and that all the unknown dissolves in the solution before you remove the test tube from the water bath. Be certain that you obtain at least six temperature readings after the rate of cooling levels off. Now weigh the remaining unknown and add to the test tube containing the BZP and initial unknown portion. Note that your sample will now contain the BZP and the total amount of unknown. Repeat the procedure described above. If your temperatures have not stabilized after 7 min, consult your instructor. Page 4 of 7 CSUS Department of Chemistry Experiment 10 Temperature vs. Time Data: (Staple to the back of your exp.) Chem. 1A Time (minutes) Pure BZP Unk. Solution 1 Unk. Solution 2 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10 10.5 11 11.5 12 12.5 13 13.5 14 14.5 Page 5 of 7 CSUS Department of Chemistry Experiment 10 Chem. 1A Data and Calculations Name: ________________________________ (Submit this and the following pages) Lab section: _________ Score: _____ /35 Mass of test tube _________________ Mass of tube + BZP _________________ Mass of BZP used _________________ Freezing point of pure BZP _________________ Using EXCEL or another graphing program, plot your temperature vs. time readings for pure BZP and for each of the two solutions. Be sure to expand your scale on the y‐axis enough so that you can easily read the temperatures to the nearest 0.1 °C. Offset the curves in time to avoid overlapping. Use the procedure described in the introduction to estimate the freezing points for use below. Unknown number ___________ Solution 1 Solution 2 Total mass of unknown used ___________ ___________ (total mass) Freezing point of solution ___________ ___________ Freezing point depression (ΔTf) ___________ ___________ Molality of unknown (eqn. 3) ___________ ___________ Molecular weight of unknown (eqn. 5) ___________ ___________ Average molecular weight of unknown ___________ Show your work below. Page 6 of 7 CSUS Department of Chemistry Data and Calculations Experiment 10 Chem. 1A Name: ________________________________ Lab section: _________ POST‐LAB QUESTIONS 1. Would the calculated molecular weight be too high, too low, or unaffected if the following occurred? (Explain your answer clearly.) a) Some BZP adheres to the test tube walls above the solution. b) Some unknown adheres to the test tube walls above the solution. c) The freezing point depression is calculated from super‐cooled BZP. d) In the calculation of molecular weight for the second determination, a student uses the mass of the second addition of unknown rather than the total mass of the two additions. (Hint: check to ensure you didn’t make this mistake!) Page 7 of 7 ...
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- Summer '11
- Chemistry, CSUS Department of Chemistry, BZP