Prof Gilbert LECTURE 16 CHEM 1211 10 18 10 My office hours in Chem Central are Tuesdays 11 noon and 1 3 and in my office 109 HT Wed noon 1 30 Additional CHEM 1211 review sessions have been organized for Monday evenings starting at 7 00 Calculating H0 rxn values from standard enthalpies of formation H0 f of the products and reactants Barometric Pressure P is the force F exerted by the weight of the atmosphere over the surface area A Announcements Reminders Last time Chapters 5 and 6 of the Earth P F A The Gas Laws PV nRT This time More Calculations 1 The volume of helium in this balloon which was flown around the world by the late Steve Fossett in 2002 was about 1 56 107 liters at 20 C and 1 00 atm of pressure What mass of helium was needed to fill the balloon 2 Determining molar mass M The number of moles n of a substance is equal to its mass m divided by its molar mass M Substituting this into fraction into the ideal gas law we have or M 3 A balloon is inflated with 4 62 g of a gas to a volume of 2 46 L at 27 C and 1 05 atm What is the molar mass of the gas 4 Gas density and molar mass A mole of any ideal gas occupies the same volume at a given temperature and pressure But the mass m of that volume and therefore the density of the gas is proportional to the molar mass M M or d M 5 What is the density of propane C3H8 at STP Would propane leaking from a tank stored in the basement of a house tend to collect in the basement or rise into the floors above 6 The tanks used in barbecue grills such as the one shown here contain about 20 pounds of propane If this much propane leaked out what volume would it occupy at 25 C and 1 00 atm 7 Mixtures of gases Rule ideal gases behave independently in a mixture each contributing its own partial pressure so that the total pressure is the sum of the partial pressures This relationship is called Dalton s law of partial pressures Ptotal P1 P2 P3 One application collecting a gas by displacing a liquid with a significant vapor pressure e g water from a jar inverted in a pneumatic trough In this example PO2 Pbar PH2O where Pbar is barometric atmospheric pressure At 25 C PH2O 23 8 mm vapor pressures increase with increasing temperature Suppose you are setting up an O2 generator like the one on the right You add 5 00 g of KClO3 to the test tube If it all decomposed how big a bottle would you need to catch all the O2 at 25 C 8 Partial pressure Px and the mole fraction Xx of gas x in a mixture Ex at 1 00 atm the values of PN2 and PO2 are 0 79 and 0 21 atm because the mole fractions XN2 and XO2 are 0 79 and 0 21 Px Xx Ptotal 9 This high performance 100 steel scuba tank weighs 7 5 pounds more when it is filled with air at 3500 psi and 25 C than when it is empty What is its volume
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