Common Gases at Room Temperature AP Chemistry Unit 5 - Gases Know these! HCN toxic slight odor of almonds HS toxic odor of rotten eggs CO toxic odorless CO odorless CH4 methane odorless, flammable CH4 ethylene ripens fruit C3H8 propane flammable NO nitrous oxide laughing gas NO toxic red, brown gas NH3 ammonia pungent SO irritating odor Atmospheric Pressure the amount of pressure created by the weight of the atmosphere on 1 m at sea level P = F/A Pressure Units Atm = 1.0135 x 10 5 Pa (N/m ) = 760 mm Hg = 760 torr Boyle s Law PV = constant P1V1 = PV closed container (n constant) temperature constant as volume decreases, pressure increases Charles Law V/T = constant V1/T1 = V/T pressure is held constant as temperature increases, volume increases
Avogadro s Law V/n = constant V1/n1 = V/n temperature and pressure are held constant as number of molecules increase, the volume increases Gay-Lusaac s Law P/T = constant P1/T1 = P/T volume is constant as temperature increases, pressure increases Ideal Gas Law PV = nrt a combination of all the gas laws R = 0.081 L atm / mol K (R = 8.314 J/mol K) Calculate the volume of an ideal gas at STP. A 0.50 mol sample of oxygen gas is confined at 0.0 C in a cylinder with a moveable piston and an initial pressure of 1.0 atm. The gas is compressed so that the final volume is half the initial volume and the final pressure is. atm.
Gas Density and Molar Mass Calculate the average molar mass of dry air if it has a density of 1.17 g/l at 1 C and 740.00 torr. (9.0 g/mol) The safety air bags in cars are inflated by nitrogen gas generated by the rapid decomposition of sodium azide (NaN3, the other product is sodium). If an air bag has a volume of 36 L and is filled at a pressure of 1.15 atm at 6.0 C, how much sodium azide must decompose? (7 g) Dalton s Law of Partial Pressures Pt = P1 + P + P3 + Pt = n1rt/v + nrt/v + n3rt/v Pt = n1 + n + n3 (RT/V) Pt = ntrt/v A gaseous mixture made from 6.00 g of oxygen and 9.00 g of methane (CH4) is placed in a 15.0 L vessel at 0.0 C. What is the total pressure in the vessel? (1.1 atm)
Partial Pressure and Mole Fraction P1 = X1Pt A synthetic atmosphere of 1.50 mole % CO, 18.0 mol % O, and 80.5 mol % Ar is created. What is the partial pressure of O if the total pressure is 745 torr? (134 torr) If the atmosphere is to be held in a 10 L space at 95 K, how many moles of O are needed? (0.87 mol O) Kinetic Molecular Theory Gases consist of large numbers of molecules that are in continuous, random motion. The combined volume of all the molecules is negligible compared to the volume of the container. Attractive and repulsive forces are negligible. Energy can be transferred between molecules during collisions, but the average kinetic energy remains constant. (Perfectly elastic collisions no friction.) The average kinetic energy is proportional to the absolute temperature. (At any temp, any gas would have the same average KE.)
Boltzmann s Equation as molar mass increases, root-mean-square (rms) speed (u, m/s) decreases u 3RT M Effusion the escape of a gas through a tiny hole Diffusion the spread of one substance throughout space or another substance Graham s Law of Effusion r1 M r M 1 r = rate of effusion M = molar mass T = temperature (Kelvin) An unknown composed of homonuclear diatomic molecules effuses at a rate that is only 0.355 times that of O at the same temperature. Identify the unknown gas. (I) Rates of Diffusion depend on the mean free path of the molecules Real Gases: Deviations from Ideal Behavior Ideal gases in the real world only occur at high temperatures and low pressure Real gases take up space and have attractions and repulsions between molecules
van der Waal s Equation takes into account real world deviations n a P V nb nrt V b = actual volume of a mole of gas (L/mol) a = accounts for decrease in pressure due to attractive forces (L atm/mol ) see table, page 49. Consider 1.000 mole of carbon dioxide gas confined to a volume of 3.00 L at 0.0 C. Calculate the ideal pressure and the pressure with van der Waal s equation. (7.473 atm, 7.18 atm) Summary Problem! Cyanogen, a highly toxic gas, is composed of 4.6% C and 53.8% N by mass. At 5 C and 751 torr, 1.05 g of cyanogen occupies 0.500 L. What is the molecular formula of cyanogen? (CN) Predict its geometry and polarity. (linear, non-polar)