Revised Molar Mass Measurement Lab Purpose: Background:

Transcription

1 Revised Molar Mass Measurement Lab (Based on prior Ch b experiment, incorporating revision suggestions from Tsze Tsang) Purpose: To use the colligative properties of cyclohexane solutions to determine the molar mass of an unknown substance with cooling curves. Background: Dissolved solutes lower the freezing and raise the boiling of solutions, and differences in solute concentration can produce an osmotic pressure gradient. Contrary to what one might expect, however, at low-to-moderate solute concentrations, all non-volatile solutes have a similar effect in this regard. To a first approximation, all that matters is the number of solute molecules/ions in solution, not the identity of solute (as long as we take ion dissociation into account!) We call such properties of a solution that depend only on the number of solute particles colligative properties. For a given solution, freezing depression is equal to the product of molality of solute particles (moles of solute / kg of solvent) and the freezing depression constant K f of the solvent (in C kg/mol). ( T f = K f x m). Which of the following solutions has a lower freezing : molal aqueous sodium chloride, or molal aqueous glucose solution? Why?. Sucrose is twice as large a molecule as glucose. Does a molal aqueous solution of sucrose show more freezing depression than a molal aqueous solution of glucose? Why/why not?. A.5-gram sample of an unknown compound is dissolved in 5. grams of benzene. The freezing of the solution is. C below that of pure benzene. What is the molar mass of the compound, assuming K f = 5. C kg/mol for benzene?

2 Summary: This experiment has three parts. First, you will find the freezing of cyclohexane in an ice bath by making a cooling curve. Second, you will find the freezing depression constant of cyclohexane by dissolving a known amount of the solute dichlorobenzene in the cyclohexane and repeating your cooling curve. Finally, you will determine the molar mass of an unknown substance by taking a third cooling curve and using the constant from your ond experiment. Wear gloves + goggles! Part : Finding the freezing of cyclohexane:. Record the total weight of an empty test tube and a piece of parafilm. (Use of a stir bar in this experiment may help, but is optional; if you decide to use one, weigh it as well.). Add 5 ml of cyclohexane to the test tube, cover the top with parafilm, and record the weight of the tube and its contents.. Set up a freezing apparatus, consisting of the test tube with a thermometer clamped to it inside a salt/ice bath.. Working in pairs, make a temperature curve. Record the temperature before the tube is placed in the ice bath, then place it in the ice bath. One partner should read the temperature aloud every onds. The other should record it below. 5. Let the cyclohexane warm up again, then repeat this experiment twice more using the same cyclohexane. Draw three freezing curves on graph paper. Do not discard the cyclohexane. Weigh the tube after to check for evaporation. Data for part : (+ attached curves) Mass of tube + parafilm (+ optional stir bar): Mass of 5 ml cyclohexane + tube + parafilm (+ optional stir bar): Mass of 5 ml cyclohexane: Temperature at different time s (in onds)

3 Part : Finding the K f of cyclohexane:. Weigh approximately mg of dicholorobenzene. Record the exact weight below.. Add the dichlorobenzene to the test tube with cyclohexane from part.. Set up the freezing apparatus as you did in part, again working in pairs. Immerse the tube in the bath, and take three freezing curves as before. (Measure temperature at and every thereafter.). Repeat this experiment two more times, then draw the freezing curve. 5. When you re done with all three runs, retrieve the magnetic stir bar (if you used one) and discard the content from the tube into the flammables waste bottle. Data for part : (+ attached curves) Mass of dichlorobenzene: Calculate molality (not molarity) of dichlorobenzene below. (MW of dichlorobenzene = 7. g/mol) Temperature at different time s (in onds)

4 Part : Finding the molar mass of an unknown substance.. Weigh an empty test tube and parafilm (+ optional stir bar), as in part.. Add 5 ml of cyclohexane to the tube, parafilm the top, and record the weight of tube + parafilm (+ optional stir bar).. Weigh out about 5 mg of the unknown substance, and record its exact weight.. Set up the freezing apparatus as before, and make three freezing curves for the unknown solution below. 5. When you re finished, retrieve the stir bar (if you used one) and dispose of waste in the flammables bottle. Data for part : (+ attached curves) Mass of tube + parafilm (+ optional stir bar): Mass of 5 ml cyclohexane + tube + parafilm (+ optional stir bar): Mass of 5 ml cyclohexane: Mass of unknown: Temperature at different time s (in onds) Molar mass of unknown substance (show calculations!):

5 Post-Lab: Write up your conclusions from each part of the experiment, with sources of error, after you answer the following questions. Don t forget to attach your freezing curves! Questions:. Look up the reported melting and freezing depression constant for cyclohexane online and compare them to your values. Calculate percent error for each.. What is the freezing of an aqueous. molal NaCl solution, assuming complete dissociation of the salt? (K f for H O =. C/molal; show your work). Suppose substance X has molar mass 5. g/mol. How many grams of substance X would you need to add to.79 kg of water to lower the freezing to -. C?