Lab: Determining the Molar Mass of Butane

Transcription

1 Lab: Determining the Molar Mass of Butane Introuction In many laboratory settings, a gas must be collecte for stuy. There are several ways of collecting an storing gases an the preferre metho will vary accoring to the gas being collecte an the purpose for which it is neee. In many occasions, chemists collect gases over water. The following iagram illustrates collecting a gas over water: In the iagram, a gas is brought in from an outsie source through a rubber tube. The tube is passe uner water until it opens into an inverte cyliner. At the start of the experiment the cyliner was completely fille with water, but as gas was collecte the water level fell. Besies the gas being stuie, water vapor also accumulates in the gas pocket. Before you can perform any calculations involving the gas sample s pressure, you must correct for the pressure of water vapor. In toay s experiment you will be collecting the butane gas from a regular cigarette lighter. After collecting it, you will make careful measurements of its mass, volume, temperature, an pressure. This will allow you to calculate the number of moles of butane trappe in the cyliner, which, along with the mass, will provie you with an experimental molar mass for butane. Pre-lab questions Possible answer: Butane is extremely flammable an will ignite. Possible answer: Because the butane will not issolve in the water, it is possible for butane gas possible to form uner water an get trappe. Possible answers: 0 C ( K) an 1 atm (101.3 kpa) C4H10 C = g/mol * 4 = g/mol H = 1.01 g/mol * 10 = g/mol MM = g/mol g/mol = g/mol Propane Methane 1000 g C3H8 = mol C3H g CH4 = mol CH g/mol C3H g/mol CH4 Methane contains the larger number of moles

2 Objective To successfully collect an store butane gas from a cigarette lighter. To use the Ieal Gas Law an Dalton s Law of Partial Pressures in calculating the number of moles of butane being store. To use the mass an the number of moles of butane being store in calculating the molar mass of butane. To compare the experimental molar mass of butane with the theoretical mass (base on its molecular formula) an calculate a percent error. Materials 1 butane cigarette lighter 1 gas-collecting bottle 1 pneumatic trough thermometer Analytical balance glass plate Safety Precautions Butane is an EXTREMELY flammable gas. At NO TIME may you ignite your cigarette lighter or have any other source of heat or flame present in lab. Proceure A. Obtain a triple-beam balance an make sure it is properly set to zero. B. Place a ry cigarette lighter on the triple beam balance an recor its mass uner Initial Mass of Cigarette Lighter in the Data an Observations section. Remember to recor the mass with all certain igits plus one uncertain igit. C. A water to a pneumatic trough (at least 2/3 full). The water level shoul be just below the spout on the sie of the trough. D. Fill a gas-collecting bottle with water to the very brim. Place a glass plate over the mouth of the bottle, then turn the bottle upsie own an place it in the trough. When the mouth of the bottle is unerwater, you may remove the glass plate. Be sure that there are no bubbles in the bottle. If the bottle has air bubbles trappe in it, you must try again to completely fill the bottle with water. E. Lift the bottle up slightly (but keep its mouth below the surface of the water) an hol the cigarette lighter beneath the mouth. Press the button on the lighter so that bubbles of butane rise into the inverte bottle. Be careful that no bubbles of butane miss the mouth of the bottle. F. As the bottle fills with butane, the level of the water will fall. Continue to collect butane gas until the water level reaches one of the marks on the sie of the bottle (for clarity, you shoul fill to the secon or thir mark rather than the first) G. When you have collecte a measurable amount of butane, release the button on the lighter an remove it from the water. Dry it with a paper towel an recor its mass again uner Final Mass of Cigarette Lighter in the Data an Observations section. The secon mass shoul be LESS than the first mass you recore in step A. If the secon mass is higher than or the same as the first mass, be sure that it is thoroughly ry (incluing insie the metal casing aroun the gas outlet) an mass it again. If the mass is STILL greater than or the same as the first mass, you shoul repeat the proceure from the beginning. H. Rea the volume of the butane you collecte from the sie of the bottle. Make sure you are reaing the correct set of measurements. Recor this volume uner Volume of Butane Collecte in the Data an Observations section. When you have recore the volume, you may set the bottle own insie the trough. I. Use a thermometer to recor the temperature of the water in the trough. You may assume that the gas temperature is the same as the temperature of the water. Recor the temperature to the nearest tenth of a egree Celsius uner Temperature of Gas Collecte in the Data an Observations section. J. The instructor will have pulle up the weather.com page with information concerning the atmospheric pressure at your location. This information will be recore on the boar in mmhg. Recor this pressure in your Data an Observations section exactly as it appears on the boar uner the heaing Total Pressure of Gas in Cyliner. K. Refer to the graph on the next page to etermine the vapor pressure of H 2O at the temperature you recore in Step I. Recor the Vapor Pressure of Water in your Data an Observations section.

3 L. Perform all calculations an conversions in the Calculations section. Don t forget to calculate a percent error when you have finishe everything else. Clean-Up Dispose of your water in the sink. Avoi inhaling the butane fumes as you empty your cyliner. Encourage the mixing of regular air with the butane sample to isperse it quickly. Carefully ry the cigarette lighter an place it on the lab table.

4 Data an Observations (Sample Data) Fining the Mass of your Gas Sample 1. Initial Mass of Cigarette Lighter: grams 2. Final Mass of Cigarette Lighter: grams Fining the Volume of the Gas Sample 3. Volume of Gas Collecte in ml: 30.0 ml Fining the Temperature of the Gas Sample 4. Temperature of Gas Collecte in ºC: 25.0 ºC Fining the Pressure of the Gas Sample 5. Atmospheric Pressure in mmhg*: mmhg 6. Vapor Pressure of Water at Measure Temperature: mmhg Calculations 7. Calculate the mass of the gas in the collecting bottle. Show your work for creit g = g 8. Convert the volume of gas collecte (Data an Observations #3) to Liters. Show your work L 30 ml/1000 ml/l = L 9. Convert the temperature recore (Data an Observations #4) to Kelvins. Show your work K 25 C = K 10. Calculate the pressure of the butane alone (#5 #6) in mmhg. Show your work mm Hg P Butane = = mm Hg

5 11. Convert the pressure of the butane gas from mmhg to atm. Show your work atm mm Hg/760 mm Hg/atm = atm 12. Calculate the number of moles of butane collecte. Use the ieal gas law an show your work mol n = (0.977 atm)(0.030 L) / ( L atm/mol K) ( K) = mol 13. Using the mass calculate in Calculation #7 an the number of moles calculate in Calculation #12, calculate the experimental molar mass of butane. Show your work g/mol MM = g / mol = g/mol 14. Butane has the molecular formula C 4H 10. Calculate its theoretical molar mass base on this formula. Show your work g/mol 15. Calculate the percent error. Show your work. +/ % % error = exp. value - theo. value x 100% theo. value % error = x 100% = +/ % Post Lab Questions (Show all work for creit): 1. Ientify at least one potential source of error in the experimental proceure that may lea to a loss of accuracy. Possible answers: Capturing gas from lighter in collection tube, Removing water that collecte in lighter before final massing, Water possibly filling in lighter while Butane gas was being expelle. 2. A gas sample is collecte over water at 20.0ºC. The volume of the gas collecte is 45.0 ml an the atmospheric pressure is 771 mmhg. How many moles of gas were collecte? (Remember to take into account the vapor pressure of water. Consult the table on Page 3 of this lab.) Calculate vapor pressure of gas collecte over water P(gas) = P(atm) P(H2O) = = mm Hg n = PV RT n = ( mm Hg/760 mm Hg/atm)(0.045 L) ( L atm/mol K) ( K) n = mol

6 3. The mass of the gas sample escribe in question 2 is foun to be grams. What is the molar mass of this gas? MM (molar mass) = g. Mol = g mol Neon = g/mol 4. It is known that the gas escribe in questions 2 an 3 is an element from the perioic table. What is the probable ientity of this mystery gas? 5. Calculate the ensity of butane at STP. MM = RT P = MM(P) RT = (58.14g/mol)(1 atm) ( L atm/mol K)( K) = 2.59 g/l Butane 6. Calculate the ensity of water vapor at 400 K an stanar pressure. = MM(P) RT = (18.02 g/mol)(1 atm) ( L atm/mol K)(400 K) = g/l 7. What volume will grams of isopentane, C 5H 12, occupy when store uner 12.0 atm of pressure an 21.0ºC? PV V = nrt = mrt P = (0.01g C 5H 12/72.17 g/mol C 5H 12)( L atm/mol K)( K) (12 atm) = L C 5H 12