EXPERIMENT 11 Measurement of the Gas Constant and Molar Volume of Oxygen GasPurpose

This experiment will allow you to gain practical experience in the collection and measurement ofthe properties of gases. From the data collected you will experimentally determine the values of theuniversal gas constant, R, and the molar volume of a gas, Vm, under standard conditions (STP)based upon your experimentally derived value of the gas constant.

Introduction

In this experiment, two values will be determined, the gas constant, R, and molar volume, Vm. Thecalculations are based on the ideal gas law:

(EQ 11.1)

where P = pressure of the gas; V = volume of the gas; n = moles of the gas; T = temperature of the gas with units of Kelvin (K); and R = the gas constant

Moles are difficult to measure directly, however the number of moles of a substance is equal to themass of the substance divided by its molecular mass so the equation above may be rearranged asbelow to replace moles by mass.

(EQ 11.2)

where m = mass and MM = molar mass.

Plugging Equation 11.2 into Equation 11.1 yields:

(EQ 11.3)

PV nRT=

MM mn---- n mMM

----------= =

PV mRTMM------------=Chemistry 120 Grossmont College 11193

Measurement of the Gas Constant and Molar Volume of Oxygen GasEquation 11.3 can now be rearranged to solve for R:

(EQ 11.4)

Rearranging Equation 11.1 an expression for molar volume, Vm, can be found:

(EQ 11.5)

In this experiment you will measure the mass, volume, temperature, and pressure of a gas to exper-imentally determine values for R and Vn. The literature values are listed below under STP condi-tions (1 atm and 0 C):

(EQ 11.6)

(EQ 11.7)

In this experiment, oxygen gas will be produced by the thermal decomposition of potassium chlo-rate in the presence of the catalyst manganese dioxide. Catalysts are substances, which increase therate of a reaction but are not used up by the reaction. Manganese dioxide functions as a catalyst bylowering the activation energy required to decompose potassium chlorate. The reaction for thedecomposition of potassium chlorate is shown below:

2 KClO3 (s) 2 KCl (s) + 3 O2 (g) (EQ 11.8)

Catalysts are not used up in the reaction, so they appear above the reaction arrow.

Procedure

Clean and dry a large (8-inch size) test tube. Weigh the test tube. Transfer 2.1-2.3 g of the KClO3/MnO2 mixture into the test tube and reweigh it. Make sure to record your mass to the precision of

Caution! Safety Notes:The KClO3/MnO2 is a powerful oxidizing mixture that mayexplode on excessive heating if the mixture is contaminated. Be

very careful not to get any glycerin in the mixture.Wear your safety goggles at all times! Especially when anyone in the labo-ratory is heating the KClO3/MnO2 mixture!

The KClO3/MnO2 waste should be disposed by dissolving the KClO3 mix-ture in water and pouring it into the inorganic substances hazardouswaste container. If solid KClO3 is put in the wastebasket it may start a fire.

PV mRTMM------------= R PV MM( )mT

-----------------------=

PV nRT= VmVn--- RT

P-------= =

R 0.08206L atmmol K---------------- 62.365

L torrmol K----------------= =

Vm 22.414L

mol---------=

MnO2

heat11194 Chemistry 120 Grossmont College

Procedurethe balance used. Remember to use a milligram balance and record all masses to at least the nearest0.001 g.

Preliminary assembly

Assemble the apparatus shown in Figure 11.1 on page 195. A correctly assembled apparatus will beon display at the instructors station as well.

1. Before attaching the test tube containing the weighed reaction mixture, use a pipet bulb to blow gently into the connecting tubing (from the end that is to be attached to the test tube) to start a siphon action between the one-liter flask and the 1000 mL beaker.

2. Siphon the water back and forth several times until you have removed all of the air bubbles. Throughout the siphoning process, take care that no water enters the short piece of tubing in the two-holed stopper that will ultimately be connected to the test tube.

FIGURE 11.1

Final assembly1. With the water level midway in the neck of the 1 liter flask, close the pinch clamp on the tube

connecting the beaker and the flask.2. Attach the short piece of rubber tubing to the glass tube sticking out of the stopper of the test

tube containing the KClO3/MnO2 mixture. Be sure that all stopper connections are airtight sothat no gas will be lost to leaks in the system.

NOTE: We will use the method of mass by difference to minimize anysystematic errors in the weighing process. Be sure to use the same bal-ance throughout the experiment!

NOTE: Because you will be collecting a gas sample, all of the tubingand stopper connections of the apparatus must be gas tight. If any part ofthe apparatus leaks during the experiment, your data will not give accu-rate results. (Also, note that a leaking apparatus constitutes a blunder

and is not just an experimental error!)

MnO2 + KClO3

Clamp

displaced waterChemistry 120 Grossmont College 11195

Measurement of the Gas Constant and Molar Volume of Oxygen Gas3. This step comprises the equalization process. In this step you are going to adjust the pressureinside of the 1 liter flask and the test tube (i.e., the system) so that it will be the same as the pres-sure on the outside, that is, the pressure in the room. Then, the pressure inside of the assembly isknown it is simply the atmospheric pressure in the room, which you will determine by readingthe barometer. The pressure of the system is to be equalized before the oxygen is produced andagain at the end of the experiment just before the volume of the displaced water is measured.

4. Take the glass delivery tube out of the beaker and empty the water out of the beaker. Handle thedelivery tube from the flask very carefully so as to not shake any water out of it. Set the beakerdown but do not dry it out with anything. Return the beaker to its position under the deliverytube and reopen the pinch clamp. From this point forward, DO NOT take the end of the deliv-ery tube out of the water that will soon flow into the beaker.

5. Only a few drops of water should flow into the beaker. If water continues to flow into the bea-ker, there is a leak in the system and you must stop, fix the leak, and repeat the equaliza-tion process before you continue. If you have a leak, make sure that the stopper is tightlyseated in the flask. Check that all of the rubber tubing-glass tubing connections are tight. Whenyou are sure they are all tight, repeat the procedure flush the system and equalize the pressurein the system.

6. At this point, BEFORE you start heating the system, you must have the system checked by theinstructor.

7. After the instructor gives you permission, make certain that the pinch clamp is open and startslowly heating the KClO3/MnO2 mixture in the tube. After 1-2 minutes of gentle heating, turnup the heat level from the burner so that the mixture melts and starts to decompose. Carefullywatch the progress of the decomposition and regulate the heat level so that there is a slow butcontinuous evolution of oxygen gas.

8. As the reaction progresses, the mixture will begin to solidify and you will have to supply moreheat. When little gas seems to be evolving, heat the tube from all sides and from the endstrongly to ensure that the last bit of KClO3 has been decomposed. At this point, you shouldhave 600 to 700 mL of water in the beaker. The end of the tube that extends almost to the bot-tom of the flask must still be submerged under the water. If the water level falls below the end ofthe tube, gas will escape and your results will be invalid.

9. When no more oxygen gas is evolved, let the system cool to room temperature. Make sure thatthe water delivery tube remains in the beaker of water and that the pinch clamp remains open.During the cooling, some water flows back into the flask to compensate for the contraction ofthe gas upon cooling.

NOTE: To equalize the system, open the pinch clamp and raise orlower the beaker till the level or height of the water in the beaker is thesame as the level or height of the water in the flask. Close the clampwhile maintaining the same water level in the flask and beaker. Again,

the purpose of this process is to ensure that the pressure in the flask is the same asthe pressure in the room, the atmospheric pressure.

NOTE: Be sure to heat slowly or the oxygen gas will be generated fasterthan it can escape through the glass tube and it will shoot the stoppersout of the test tube or the flask, meaning you must start over! If thewater level in the flask gets close to the bottom of the glass tubing that

leads to the beaker, immediately stop heating the test tube. If the water level low-ers past the bottom of the glass tubing you must start over!11196 Chemistry 120 Grossmont College

Calculation Hints10. While you are waiting for the cooling, measure the barometric pressure in the room. This mea-surement should have four significant figures. Your instructor will show you how to do this.

11. When the system has cooled to room temperature, equalize the pressure and while the waterlevels are the same in the beaker and the flask, close the pinch clamp. Be sure you have thepinch clamp securely in place before you do this next step!

12. Record the temperature of the gas in the flask, to one decimal place ( 0.1 C), by releasing thestopper slightly and inserting a thermometer into just the gas. After the temperature comes toequilibrium, measure the temperature of the water remaining in the flask to one decimal place.

13. When it is cool, weigh the test tube and KCl/MnO2, residue. Measure the volume of water dis-placed into the beaker with a 1000 mL ( 5 mL) graduated cylinder.

TABLE 11.1 Vapor Pressure of Water

Calculation Hints1. From the mass of the oxygen liberated, calculate the number of moles of O2 produced. The pres-

sure inside the system is equilibrated to atmospheric pressure. As the oxygen is collected overwater it is saturated with water vapor, so

Pbar = Poxygen + Pwater vapor (EQ 11.9)

The water vapor pressure at a variety of temperatures can be found in Table 11.1 on page 197.From the barometric pressure (i.e. the atmospheric pressure) and the water vapor pressure, cal-

T (oC) Vapor Pressure of Water (mm Hg)

10 9.20911 9.84412 10.51813 11.23114 11.98715 12.78816 13.63417 14.53018 15.47719 16.47720 17.53621 18.65022 19.82723 21.08824 22.37725 23.75626 25.20927 26.73928 28.34929 30.04330 31.824Chemistry 120 Grossmont College 11197

Measurement of the Gas Constant and Molar Volume of Oxygen Gasculate the pressure of the O2. From the volume of water displaced, determine the volume of O2produced.

2. Calculate the value of the gas constant, R, by substituting the known quantities from you mea-surements into the ideal gas equation and solving for R.

3. Evaluate the molar volume, Vm, by using the value of R just calculated and the standard temper-ature and pressure (STP) conditions in the ideal gas equation.

4. Compare both your value for R and for Vm to the accepted values. This comparison should con-sist of percentage error calculated by:

(EQ 11.10)

Be sure to show the positive or negative sign. What does the sign tell you about your percent error?

5. Calculate the percent of KClO3 in the original mixture form the amount of oxygen gas pro-duced:

(EQ 11.11)

% error observed value accepted valueaccepted value

----------------------------------------------------------------------------- 100% =

% KClO3mKClO3mmixture------------------- 100% =11198 Chemistry 120 Grossmont College

Prelaboratory ExercisePrelaboratory Exercise

Gas Constant and Molar Volume

Name: __________________________

Section:____________________

1. Write the balanced equation for the production of the oxygen gas in this experiment.

2. How much of the KClO3 mixture are you to use in you experimental work?

Between ______________ and ______________ grams

3. In this experiment MnO2 is used as a catalyst. What is the function of the MnO2 in this reaction?

4. After you have set up your apparatus as shown in Table 11.1 on page 195 and you think that thesystem is air tight, what are you supposed to do next (i.e., before actually starting the experi-ment)?

5. Before you start displacing the water with the oxygen and again when you are all done, youmust equalize the pressure inside of the system so that it is the same as that in the room.

a. Why is this necessary?

b. How is it done?

Lab Grade

Prelab Questions 10

General Format (Signature, ink, no obliterations, etc.)

15

Data and Analysis (observations, questions, units, significant fig-ures, sample calculations, etc.)

20

Accuracy (% error) 30

Post Lab Questions 25

Total 100Chemistry 120 Grossmont College 11199

Measurement of the Gas Constant and Molar Volume of Oxygen Gas6. A sample of oxygen gas is collected over water using the same procedure you will use in thisexperiment. The system is equalized and the temperature of both the water and the gas is 22oC.The atmospheric pressure is 745 mm Hg, what is the pressure of the O2 gas in the flask?

7. Use the data below to answer the following:TABLE 11.2

Sample Calculations:

Mass of empty test tube 18.767 gMass of test tube + sample 20.989 gMass of test tube assemble + residue 20.164 gMass of O2, liberated

Volume of H2O displaced 675 mLVolume of O2 evolved

Temperature of O2 in flask 24.0 CTemperature of H2O left in the flask 23.9 CBarometric pressure 742.0 torrVapor pressure of water

Calculated pressure of O2 gas

Calculated Value of R in L atm/mol K11200 Chemistry 120 Grossmont College

Data Sheet: Gas Constant and Molar VolumeData Sheet: Gas Constant and Molar VolumeTABLE 11.3

Sample Calculations

Mass of empty test tube Mass of test tube and KClO3/MnO2 Mass of KClO3/MnO2 mixtureMass of test tube assembly and residue Mass of O2, liberated Volume of H2O displaced Volume of O2 liberatedTemperature of O2 in flask Temperature of H2O left in the flask Barometric pressure Vapor pressure of water Calculated pressure of O2 gas

Experimental Value of RLiterature Value of R 62.4 L torr/mol KPercentage error in RMolar volume, VmLiterature Value of Vm 22.4 L/molPercentage error in VmCalculated amount of KClO3 Percentage of KClO3 in the original mixtureChemistry 120 Grossmont College 11201

Measurement of the Gas Constant and Molar Volume of Oxygen Gas11202 Chemistry 120 Grossmont College

Post Lab QuestionsPost Lab Questions1. How would your result (i.e., the R value) be affected if you did not wait for the test tube to cool

before measuring the volume of water? Explain your answer.

2. One mole of gas is collected over water (i.e. volume of the dry gas) at a total pressure of 750mm of Hg and 30C.

a. What is the pressure of the dry gas?

b. What is the volume of the dry gas under the experimental conditions?

3. A mixture of gases at 760 mm Hg pressure contains 65.0% nitrogen, 15.0% oxygen, and 20.0%carbon dioxide, by volume. What is the partial pressure of each gas in mm of Hg?Chemistry 120 Grossmont College 11203

Measurement of the Gas Constant and Molar Volume of Oxygen Gas4. What error would result from incomplete (say only 80%) decomposition of the KClO3 due toinadequate heating? Explain your answer.

5. Assume that 1.40 g of a KClO3 /MnO2 mixture (65% KClO3, by weight) was decomposed in theabove experiment and all final measurements were made at 22C on a day when the atmo-spheric pressure was 716 torr.

a. What is the pressure of the O2?

b. How many moles of O2 should have been evolved?

c. What volume of O2 would be evolved under the experimental condition?

d. What is the STP volume of this O2?11204 Chemistry 120 Grossmont College