DETERMINATION OF THE MOLAR MASS OF CARBON DIOXIDE

Transcription

1 DETERMINATION OF THE MOLAR MASS OF CARBON DIOXIDE INTRODUCTION Molar masses of gases can be determined in a number of ways, many of them indirect. In this experiment you will find the molar mass directly, using a measured mass of a dry CO 2 sample, its volume, its temperature, its pressure, and the Ideal Gas Law. You will generate your CO 2 sample from the action of hydrochloric acid on marble chips, a form of calcium carbonate. CaCO 3(s) + 2HCl (aq) CaCl 2(aq) + H 2 O (l) + CO 2(g) You will do this in the apparatus shown below. CaC1 drying tube 2 Flask 1 Flask 2 Figure 1 Carbon dioxide will be generated in flask 1 from the addition of 6M hydrochloric acid to marble chips covered with water in the bottom of the flask. The continuously generated gas will be forced from the flask into a drying tube filled with anhydrous calcium chloride pellets which will absorb any water carried by the CO 2. The dry CO 2 leaves the drying tube and flows through flask 2, flushing out air and carbon dioxide as it does so. The arrows on the apparatus show the path of CO 2 flow. The flow of CO 2 through flask 2 will be continued for 10 minutes in order to make sure that there is only CO 2 in flask 2. At this point, you will collect the data needed to calculate the molar mass of the gas as follows:

2 Pressure of the Gas Sample (P) Since the gas is being generated in a generator open to the atmosphere, the pressure of the gas sample will be the same as the atmospheric pressure measured with a barometer. Temperature of the Gas Sample (T) The gas temperature will be determined in degrees Celsius using a conventional thermometer. This temperature will be converted to Kelvin for use in the Ideal Gas Law. Volume of the Gas Sample (V) Since gas samples always fill their containers, the volume of the gas sample will equal the volume of flask 2 plus the volume of its stopper assembly (stopper and both glass tubes). You will determine the volume of the flask-stopper system by filling the flaskstopper system with water and weighing it. By subtracting the weight of the flaskstopper system filled with air from the flask-stopper system filled with water, you obtain the weight of the water in the system. (Actually this subtraction gives you the weight of water in the flask minus the weight of the air included in the weight of flask-stopper system. However, the weight of the water filling the flask is so much greater than the weight of air that previously filled the flask, that the weight of the water minus the weight of the air is essentially equal to the weight of the water. The weight of the air filling the flask is essentially negligible when subtracted from the weight of the water filling the flask.) The density of liquid water is l.00 g/ml. The weight of a water sample in grams then equals its volume in milliliters. Thus the weight of the water filling the flask-stopper system equals the volume of the flask-stopper system, or the volume occupied by the gas sample. Weight of the Gas Sample (g) You will obtain the weight of the CO 2 sample by subtracting the weight of the empty flask-stopper system from the weight of the flask-stopper system filled with CO 2. The weight of the empty stopper-flask system is a little tricky to find, since all of the weights of the stopper-flask system have included weight of water, air or CO 2. You will have to find the weight of the empty flask indirectly. One way to do this is to find the weight of the air in the flask-stopper system when you weighed it and subtract it from the weight of the flask-stopper system filled with air. To find this weight of air, you will take advantage of the relationship between density, volume and weight. mass density= volume (Eq. 1) When utilizing this relationship for a gas, both the volume and the density of the gas sample must be known at the same temperature and pressure. You will know the volume of air at a nonstandard T and P and the density of air at STP (standard temperature and pressure (1.29 g/l at STP). You will have to convert one of these parameters (density or volume of air) to the conditions of the other parameter to find the correct mass of the air

3 in the flask-stopper system. Subtraction of the mass of air from the weight of the flaskstopper system filled with air gives the weight of the empty flask-stopper system. Subtraction of this weight from the weight of the flask-stopper system filled with CO 2 gives you the weight of the CO 2 sample collected. Using one of two commonly used forms of the Ideal Gas Law, it should be a simple task to calculate the molar mass of CO 2 : M = mrt (Eq. 2) PV where M is molar mass, m is mass in grams, and the other symbols have their usual significance, or P M = drt (Eq. 3) where d is gas density ( defined as mass over volume) and the other symbols have their usual significance. PROCEDURE The apparatus you will use is sketched on the first page of this handout. Flask 2 is a clean and dry 125-mL Erlenmeyer flask equipped with a 2-hole rubber stopper containing two tubes bent at right angles. One of these tubes extends to the bottom of flask 2, the other extends to just below the bottom of the stopper. Attach a drying tube, filled with chunks of anhydrous calcium chloride held loosely in place by plugs of cotton, to a ring stand. Connect the bulb end of this drying tube to the longer bent tube in flask 2, using a short piece of rubber tubing. Then attach the shorter of flask 2 s bent tubes to the side arm of a water aspirator with a rubber tube and turn on the water full force. The water rushing through the aspirator creates a partial vacuum that pulls air through the drying tube and through flask 2. Let this continue for five minutes. The purpose of this step is to make certain that flask 2 is filled with dry, room temperature air. While this is being done, prepare generator flask 1 using a second 125-mL flask as the generator, and add approximately 30 grams of marble chunks. Insert a 2-hole rubber stopper containing a thistle tube and a short, right angle tube. The thistle tube must extend to near the bottom of the flask. Add approximately 20 ml water to the flask so that the bottom of the thistle tube is covered. The generator flask 1 is now ready for the experiment. By this time, flask 2 is ready as well. Disconnect flask 2 from the aspirator and from the drying tube. Immediately, to prevent any moist lab air from entering the flask, plug the two tubes in flask 2's stopper with rolled up plugs of plastic film (Parafilm). Weigh flask 2, its stopper, glass tubes and two plastic film plugs to the nearest milligram. (Keep the plastic plugs for subsequent weighings). From this point on, do not use your hands to handle flask 2. Handle with crucible tongs, so as not to add the weight of oils and crud from your fingers to this accurately known weight. Record this weight in the Data and Calculation Sheet.

4 Attach generator flask 1 to the drying tube to flask 2 as shown in the diagram. Be sure that all stoppers are tight. Ask your instructor to check your apparatus before going on. Begin to add 6M HC1 to the generator flask through the thistle tube. A 10 ml portion is sufficient for a start. Foaming indicates that CO 2 is being formed. You must not allow the generator to stop producing CO 2 because you do not want any air to diffuse back into flask 2 through the open tube on flask 2's stopper. You can verify that CO 2 is still being produced by covering the open tube on flask 2 with your finger. If CO 2 is being generated, liquid will rise up the thistle tube. Check frequently for CO 2 generation, and add more 6M HCl as needed. After generating dry CO 2 and passing it through flask 2 for at least 15 minutes, disconnect flask 2 and plug the two tubes with your plastic film plugs. Weigh flask 2, its stopper assembly and its CO 2 to the nearest milligram and record this in your notebook. After removing the plastic plugs, reattach flask 2 to the generator flask, and generate dry CO 2 for another 10 minutes. At the end of this time, again disconnect flask 2, replug its tubes and weigh it again. The two successive CO 2 flask weighings should differ by no more than 4 mg (0.004g). If they differ by more than 4 mg, generate more CO 2 through flask 2 until two successive weighings of flask 2 filled with CO 2 differ by no more than 4 mg. At this point remove the stopper from flask 2 and measure the temperature of the CO 2. Also obtain the barometric pressure for the day. Record these data in the Data and Calculation Sheet. Finally, fill flask 2 to the brim with water. Insert the stopper with its glass tubes tightly into the flask. With an eyedropper fill the glass tubes with water and plug them. There should be no air bubbles anywhere in your apparatus. Weigh flask 2 and its stopper full of water to the nearest hundredth of a gram and record this mass in the Data and Calculation Sheet. Safety and Waste Disposal 1. If you spill any 6M HC1 on yourself, wash immediately with running water. Report burning sensations to your instructor. 2. When dismantling your generator flasks, pour the HC1 solution into a sink, flushing it well with running water. Rinse the marble chunks well with water, and discard the washed chunks into a wastebasket. 3. Cover both ends of the CaCl 2 drying tube with parafilm. If the tubes and the CaCl 2 have gotten wet from water splashing through them pour out the CaCl 2 into the sink, and dissolve it in running water.

5 CALCULATIONS 1. Mass of water in flask - subtract the mass of flask, stopper and air from the mass of the flask, stopper and water. 2. Since the density of water is 1.00 g/ml, the volume of water in the flask will be the same numerical value as the mass of water (calculation 1), but with units of ml. Show the conversion using density as a conversion factor. 3. The volume inside the flask is the same regardless of whether a liquid or a gas is contained in the flask. So the volume of gas in the flask is the same as the volume of water in calculation step 2. PV PV 4. Use the combined gas equation ( 1 1 = 2 2 ) to calculate the volume of air in the flask nt 1 1 nt 2 2 at STP. (NOTE: values of n cancel out since the number of moles of gas remains constant.) 5. Given as 1.29 g/l of air at STP. 6. Use the density of air in #6 above as a conversion factor to convert from volume of air in flask (calculation 4 above) to mass of air in flask. 7. Subtract the mass of air (calculated in step 6 above) from the mass of flask, stopper and air (Data section, item 1). 8. Subtract the mass of the empty flask and stopper (calculation 7 above) from the mass of flask, stopper and CO 2 (greatest mass obtained in Data section) to obtain the mass of just the CO 2 in the flask. 9. Use Eq. 1 to obtain the density of gas to use in Eq. 3. Be sure to use the volume obtained under laboratory conditions for this calculation. 10. Calculate the molar mass of CO 2 using either Eq. 2 or Eq experimental value - accepted value percent error = x 100 accepted value

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7 DETERMINATION OF THE MOLAR MASS OF CARBON DIOXIDE Data and Calculation Sheet Name Data: 1. Mass of flask + stopper + air g 2. Mass of flask + stopper + CO 2 (1st weighing) g 3. Mass of flask + stopper + CO 2 (2nd weighing) g 4. Mass of flask + stopper + H 2 O g 5. Temperature of CO 2 in flask o C 6. Atmospheric pressure mm Hg Calculations: (show your work on a separate sheet of paper) 1. Mass of H 2 O in flask g 2. Volume of H 2 O in flask ml 3. Volume of gas in flask - lab conditions ml 4. Volume of air in flask at STP ml 5. Density of air at STP 1.29 g/l 6. Mass of air in flask g 7. Mass of empty flask and stopper g 8. Mass of CO 2 in flask g 9. Density of CO 2 in flask g/l 10. Molar mass of CO 2 g/mole (use flask vol. at lab conditions, not at STP) 11. Percentage error

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9 DETERMINATION OF THE MOLAR MASS OF CARBON DIOXIDE Pre-Lab Assignment Name 1. Write a balanced equation for the reaction that takes place in generating flask 1. 2a. How many liters of dry CO 2 gas measured at STP can be produced from 30.0 g of CaCO 3? b. How many liters would this amount of gas occupy at 745 mm Hg and 22 o C? 3. What is the molar mass of an unknown gas if g of the gas in a L container exerts a pressure of atm at 42.5 o C?