Name Period Date. Lab 5: The Molar Volume of a Gas

Transcription

1 Name Period Date Lab 5: The Molar Volume of a Gas Objective: To determine the actual molar volume of oxyen as To determine an experimental value for the universal as constant Introduction To calculate the molar volume of a as, you must create a as and determine the volume and number of moles of as created. To do this, you will be heatin a mixture of potassium chlorate and mananese dioxide (a catalyst). Catalysts are substances which increase the rate of a reaction but are not used up by the reaction. Mananese dioxide functions by lowerin the temperature required to decompose the potassium chlorate. When heated to a temperature between 80 and 400 C all of the potassium chlorate is decomposed to potassium chloride and oxyen as. The only reaction that will occur under the conditions of the reaction is the followin: 2 KClO (s) 2 KCl (s) + O 2 () The oxyen formed will be collected as a as. The mananese dioxide and potassium chloride formed durin the reaction will remain in the test tube. Notice that the stoichiometric ratio of potassium chlorate to potassium chloride is one to one. Therefore the difference in weiht between the potassium chlorate and the resultin potassium chloride must all be due to the oxyen. Therefore, the loss in mass of the mixture in the test tube durin the reaction should equal the mass of oxyen collected. From the mass of the oxyen as produced, the number of moles of oxyen as can be calculated. The atmospheric pressure (total pressure in the jar) can be determined usin a barometer but the pressure of the dry oxyen as must be calculated usin Dalton s Law of Partial Pressure. The pressure of water vapor at the temperature at which the water is can be determined from a chart. The volume of as produced is equal to the amount of water that was displaced and pushed throuh the rubber tubin into the beaker by the end of the experiment. To be able to compare our experimental value to the standard molar volume (22.4 L/mol), we must determine the molar volume at STP (0 C, 1atm). We can use the combined as law to find the volume the sample of oxyen as created would have at STP. Be sure to use the pressure of the dry oxyen as (not atmospheric pressure) as your oriinal pressure. From the volume at STP and moles of oxyen as, the molar volume can be calculated: From the pressure of dry oxyen as, moles of as created, volume of as collected, and temperature of the water, the universal as constant, R will be calculated. Usin the Ideal Gas Law: 1

2 Materials potassium chlorate, KClO (s) mananese (IV) oxide, MnO 2(s) rin stand utility clamp lare test tube lass jar 250-mL beaker 100-mL raduated cylinder Bunsen burner Glass elbows, 2 Glass tubin, straiht pieces, 2 Rubber tubin, 2 analytical balance Rubber stoppers, 2 LabQuest Temperature sensor Gas pressure sensor Straw pieces Safety Concerns Potassium chlorate is a powerful oxidizin aent and reacts readily with easily oxidized substances such as rease or rubber. Careless handlin of it could conceivably result in a fire. Safety lasses and aprons should be worn at all times. Procedure 1. Preparation a. Set up the apparatus as shown in the fiure below except for the test tube with its rubber stopper. The lon lass tubin in the lass jar should extend almost to the bottom of the flask. Fill the jar with water and add approximately 100 ml of water to the beaker. b. Displace the air in the rubber tubin that leads to the beaker by steadily blowin into the rubber tubin, which will be connected to the test tube. Cut straws are supplied so that you need not put your mouth directly to the tubin. Remove any remainin air bubbles in the rubber tubin that leads to the beaker by raisin or lowerin the beaker. Finish with the lass jar nearly filled with water (one inch below the rubber stopper) and then place a clamp on the rubber hose that leads to the beaker. c. Find the exact mass of your empty, dry test tube. Record it in your data table. d. Weih out approximately 0. rams of KClO. Be sure that there are no lumps in it. Usin the waxed weihin paper, pour this into the test tube, bein 2

3 careful to not allow any particles to stick to the side near the top of the test tube. Record the exact mass in your data table. e. Next, weih out approximately 0.02 rams of MnO 2. Add this in the same manner to the test tube. Mix the KClO and the MnO 2 toether by swirlin the test tube ently. f. Attach the test tube to the remainder of the assembly. Incline the test tube upward at an anle of about 0 o.. Equalize the pressure inside the lass jar with the atmospheric pressure as follows: With the clamp open raise the beaker until the water level in the beaker is the same as that in the lass jar. Durin this process, do not allow the tubin in the beaker to come above the water level (or air will et into the system). Close the clamp on the tubin aain. Empty the beaker of any water and dry it. h. Place the beaker back on the desk and open the clamp. If there are no leaks in your setup, a small amount of water may flow into the beaker but it should stop quickly. Leave this water in the beaker- it will flow back into the jar at the end of the experiment when the pressure is equalized aain. If a leak is present check the connections until you have found and corrected the leak. 2. Create the Oxyen Gas a. Open the pinch clamp. Leave in the beaker any water that may flow. Heat the test tube startin at the top of the salt mixture and workin downward, at a rate that ives a moderate, steady rate of evolution of oxyen without propellin the mixture up the test tube toward the rubber stopper. b. Once the reaction has started and water beins to flow make sure that Tube F stays under water in the jar or air will et into the system and cause the calculated volume of as to be wron. Continue heatin for 15 minutes. c. Allow the apparatus to cool to room temperature.. Measurements concernin the Gas a. Measure the barometric pressure usin pressure sensor and record it in your data table. b. When the apparatus is cool to the touch (which will require several minutes) equalize the pressure aain (see #1) and then close the pinch clam. c. Remove the test tube from the apparatus and find the mass of it. Record the exact mass on your data table. d. Remove the stopper from the lass jar, insert the temperature sensor into the water inside, and leave for a few minutes. Record the temperature (as the temperature of the as). e. Look up the vapor pressure of water at this temperature in the Table iven and record it in your data table. f. Measure the volume of water in Beaker by pourin it into a raduated cylinder. Read the volume to the nearest tenth and record in your data table. This is equal to the volume of oxyen as that was created. 4. Disposal: Dispose of the residue in the test tube by washin it down the sink with lots of water. 5. Repeat the collection: Repeat Steps 1-4 to collect a second sample of as.

4 Data/Results: Mass of empty test tube Run 1 Run 2 Mass of the KClO Mass of the MnO 2 Calculate mass of test tube and KClO /MnO 2 Mass of test tube and residue after heatin Calculated mass of O 2 liberated Volume of H 2 O in beaker (Volume of oxyen created) ml ml Temperature of H 2 O in jar K K Barometric pressure mmh mmh Vapor pressure of water mmh mmh Calculated pressure of dry O 2 as mmh mmh Calculations: For each of the calculations below, show all steps of your work clearly. Be sure to follow all rules concernin sinificant fiures and roundin. 1. From the mass of the oxyen as collected, calculate the number of moles of oxyen as collected. 2. From the volume, temperature (in Kelvin) and pressure (of dry oxyen as) of the collected as, calculate the volume the oxyen as would have at STP usin the combined as law. 4

5 . Usin the number of moles of as and the volume of that as at STP, calculate the molar volume of oxyen as. Averae: 4. Calculate the percent error in the averae of your two molar volume values and the Standard Molar Volume for an ideal as. 5. Usin the pressure (of dry oxyen as), volume, number of moles, and temperature of the collected as, calculate the ideal as law constant, R. Be sure to use the correct units to end up with an R in L. atm/mol. K. Averae: 6. Calculate the percent error in the averae of your two R values and the value of R used commonly. 7. Students performin this lab have made several mistakes in the past. How would each of these oversihts affect the calculated molar volume of as? a. Forot to empty and dry out the beaker after settin up the equipment, removin air from the lines, and equalizin the pressure. b. Forot to mix the potassium chlorate and mananese dioxide toether before heatin. 5