CHAPTER 5 AP CHEMISTRY

PRESSURE Barometer Measure the atmospheric pressure Gas laws Boyles law P1V1 = P2V2 A gas that obeys the gas laws (look law up in book) is an Ideal Gas Charles's law V1 = V2 T1 T2 A gas cannot have a negative volume. Therefore, temperature can never be below 0K. 0K will never be reached. Closest is 0.000001 K Barometer Measure the atmospheric pressure Atmospheric pressure Mass of the air being pulled toward the center of the earth by gravity Manometer - on the board 1 standard atm. = 760 mmHg = 760 torr = 101.325 kPa

CONTINUED Avogardro’s law Volume is directly proportional to the number of moles V1 = V2 n1 n2 Ideal gas law PV = nRT R = 0.08206 L.atm/ K.mol Combined gas law P1V1 = P2V2 T1 T2

PROBLEMS A sample of nitrogen gas has a volume of 1.75 L at STP. How many moles are present? PV = nRT Find the R value, all units must be correct A sample of methane gas having a volume of 2.80 L at 25 oC and 1.65 atm was mixed with oxygen gas having a volume of 35.0 L at 30 oC and 1.25 atm. The mixture was ignited to form carbon dioxide and water. Calculate the volume of CO2 at a pressure of 2.50 atm and temperature 125 oC. temperature must be in Kelvin. Use the ideal gas law to find the # of moles of each reactant and get a balanced equation A sample of diborane gas (B2H6), a substance that bursts into flame when exposed to air, has a pressure of 345 torr at a temperature of -15 oC and a volume of 3.48 L. If conditions are changed so that the temperature is 36 oC and the pressure of 468 torr. What will the volume be? use the combined gas law

MOLAR MASS OF GAS Molar mass (MM) = dRT P d = density Density of a gas at 1.50 atm and 27 oC is found to be 1.95 g/L. Calculate the molar mass of the gas. Partial pressure Total pressure exerted is the sum of ALL gas pressures Ptotal = P1 + P2 + ........ = ntotal(RT/V)

MOLE FRACTION Ratio of the number of moles to the total number of moles X1 = n1/ntotal = P1/Ptotal If a gas is collected by displacing water then the gas obtained would be a mixture of the gas and water vapor. The water vapor pressure would need to be removed before you could use the gases pressure. Find the water vapor pressure at the temperature given and subtract it from the total pressure

KINETIC THEORY OF GASES The volume of individual particles are assumed zero Particles are in constant motion Particles exert no force on each other Average kinetic energy is directly proportional to its Kelvin temperature Real Gases have finite volumes and do exert forces on each other (KE)average = 3/2 RT Kelvin temperature is an index of the random motions of the particles of gases Root mean square velocity (special kind of average) Urms = (3RT/M)1/2 U = m/s, R = 8.3145 J/K.mol, J = kg.m2/s2, M = kg/mol Calculate the root mean square velocity for atoms in a sample of helium gas at 25 oC.

EFFUSION AND DIFFUSION REAL GASES Passage of a gas through a tiny orifice into a evacuated chamber Rate of effusion for gas 1 = UrmsX1 = (M2)1/2 Rate of effusion for gas 2 Urms X2 (M1)1/2 Molar mass (g/mol) is M in this formula Diffusion Mixing of gases Real gases Ideal gas behavior only occurs for real gases under the following conditions Low pressure and/or high temperature

CONTINUED Van der Waal equation [Pobs + a(n/V)2] x (V - nb) = nRT Pobs = observed pressure, a(n/V)2 = pressure correction, (V - nb) = corrected volume A and b are varied until the best fit of the observed pressure is obtained. These are found on table 5.3 Read 5.10 UNDERSTAND THIS SECTION