Experiment 11MOLAR MASS OF A VOLATILE LIQUID One of the important applications of the Ideal Gas Law is found in the experimental determination of the molarmasses of gases and vapors. In order to measure the molar mass of a gas or vapor we need simply to determine themass of a given sample of the gas under known conditions of temperature and pressure. If the gas obeys the IdealGas Law, PV = nRT (1) If the pressure P is in atmospheres, the volume V in liters, the temperature T in K, and the amount n in moles,then the gas constant R is equal to 0.0821 L ATM/(mole K). From the measured values of P, V, and T for a sample of gas we can use Equation I to find the number of molesof gas in the sample. The molar mass in grams, MM, is equal to the mass g of the gas sample divided by thenumber of mole n.n = PV Molar Mass = grams (2) RT No. of Moles This experiment involves measuring the molar mass of a volatile liquid by using Equation 2. A small amount ofthe liquid is introduced into a weighed flask. The flask is then placed in boiling water, where the liquid willvaporize completely, driving out the air and filling the flask with vapor at barometric pressure and the temperatureof the boiling water. If we cool the flask so that the vapor condenses, we can measure the mass of the vapor andcalculate a value for MM. WEAR YOUR SAFETY GLASSES WHILE PERFORMING THIS EXPERIMENTEXPERIMENTAL PROCEDURE Obtain a 600-mL Florence flask, a stopper, a cap (2 cm square of aluminum foil), and anabout 10-mLs of unknown liquid from the main workbench. If you should find a crack in theflask, report it to your instructor immediately so that it can be replaced. With the aluminumfoil cap loosely covering the neck of the Florence flask, weigh the empty, dry flask on theanalytical balance.Pour about 5-mL, about half your unknown liquid, into the flask. Assemble the apparatus asshown in Figure 1. Place the cap on the neck of the flask. Observe the liquid level in your flask.Add a two or three, at most, boiling chips to the water in the 600-niL beaker and heat the waterto the boiling point. Watch the liquid level in your flask; the level should gradually drop asvapor escapes through the cap. After all the liquid has disappeared and no more vapor comesout of the cap, continue to boil the water gently for approximately 5. Measure the temperatureof the boiling water. Shut off the burner and wait until the water has stopped boiling (about 1/2minute) and then loosen the clamp holding the flask in place. 11- 1 FIGURE .1Remove the flask from the beaker of water, using paper-toweling hold by the Florence flask bythe neck. To cool the Florence flask and the vapors, immerse the flask in a beaker of coolwater to a depth of about 5-cm. After holding the flask in the water for about 2 minutes. (Asthe flask cools the vapor inside condenses and air rushes into the flask.)Dry the flask with a towel to remove the surface waters and let it cool to room temperature.Reweigh the flask. Read the atmospheric pressure from the barometer.Repeat the procedure using the remaining 5-mL of your liquid sample.You may obtain the volume of the flask by measure its volume. Weigh the dry empty Florencewith the aluminum cap on a top-loading balance. Then fill the Florence flask with deionizedwater and again weigh on a top-loading balance.When you have completed the experiment, return the Florence flask to the main workbench. 11- 2 Data and Calculation:Molar Mass of a Volatile LiquidName _____________________ Instructor ___________ Date _______ Trial 1 Trial 2Unknown no.Mass of flask and stopper ________________g _______________gMask of flask, stopper, and ________________g _______________gcondensed vaporMass of flask, stopper, and water ________________g _______________gTemperature of boiling water bath _______________oC ______________ oCBarometric pressure ___________mm Hg ___________mm HgCalculations and Results ______________atm _____________atm ________________L _______________LPressure of vapor, P ________________K _______________KVolume of flask (volume of vapor), V ________________g _______________gTemperature of vapor, T K ____________moles ____________molesMass of vapor, gNumber of moles of vapor, nMolar mass of unknown, as found bv ________________g _______________gsubstitution into Equation 2Average Molar Mass of unknown ___________ grams 11- 3 Advance Study Assignment: Molar Mass of a Volatile LiquidName _____________________ Instructor ___________ Date _______ 1. A student weighs an empty flask with the aluminum cap and finds the mass to be 55.4415 g. She then adds about 5-mL of an unknown liquid and heats the flask in a boiling water bath at 100oC. After all the liquid is vaporized, she removes the flask from the bath, and lets it cool. After it is cool, she weighs the flask and the condensed vapor, obtaining a mass of 56.0397 g. She then drys the flask and fills it with deionized water weights the flask and water and obtains a total weight of 271.241 grams. The barometric pressure in the laboratory that day is 752.7 mm Hg and the temperature is 19.5oC. The density of water at 19.5oC is found in the handbook of Chemistry and Physics to be 0.996532 g/mL. a. What was the pressure of the vapor in the flask in atm? P = ______ atm b. What was the temperature of the vapor in K? T = ______ Kc. The volume of the flask in liters? V = _______ Ld What was the mass of vapor that was present in the flask? mass = _______ gramse. How many moles of vapor are present?f. What is the mass of one mole of vapor? (Eq. 2.) n = _______ moles 11- 4 MM = _________ g/mole 2. How would each of the following procedural errors affect the results to be expected in this experiment? Give your reasoning in each case. a. All of the liquid was not vaporized when the flask was removed from the water bath. b. The student continued to heat the apparatus for 30 minutes after all the liquid had evaporated. c. The flask was not dried before the final weighing with the condensed vapor inside. d. The flask was left open to the atmosphere while it was being cooled while the student took a 30-minute coffee break just before the final weighing. e. The flask was removed from the bath before the vapor had reached the temperature of the boiling water. All the liquid had vaporized. 11- 5
Experiment 11 MOLAR MASS OF A VOLATILE LIQUID
Description: 11- 1 Experiment 11 MOLAR MASS OF A VOLATILE LIQUID One of the important applications of the Ideal Gas Law is found in the experimental determination of the molar
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