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Wkst 6: Study Guide - Gases Objectives: 1. Use Boyle’s Law, Charles’ Law, Avogadro’s Law, and the combined gas law, as appropriate, to calculate changes in pressure, volume, temperature, and amount of gas. 2. Use the ideal gas law equation to do calculations about a sample of gas. 3. Do calculations involving gas densities and the standard molar volume. 4. Determine molecular weights and formulas of gaseous substances from measured properties of gases. 5. Carry out calculations with gaseous compounds involved in chemical reactions (volume, mass, # of moles, etc.) 6. Use the law of partial pressure to calculate the pressure exerted by gases in a mixture (including gases collected over a liquid; factor in vapor pressure of the liquid) 7. Describe the kinetic-molecular theory of gases, and how this theory is consistent with the observed gas laws. Equations: Combined gas law: P1V1 / T1 = P2V2 / T2 Ideal gas law: PV = nRT, where R = 0.0821 Latm / molK Partial pressure: Graham’s Law of Effusion: Ptotal = P1 + P2 + P3 + … Pgas = Patm – PH2O rate1 = rate 2 MM 2 MM 1 Problems: 1. A 250.0 mL volume of gas is collected at 30.0C. What volume would the sample of gas occupy at 20.0C, assuming constant pressure? 2. A 3.00 L pocket of air at sea level has a pressure of 1.00 atm. Suppose the air pocket rises in the atmosphere to a certain height and expands to a volume of 10.0 L. What is the pressure of the air at the new volume? (assume constant temperature) 3. A weather balloon has a volume of 1750. L at 1.10 atm. The balloon is released into the atmosphere. At the highest point above the ground, the pressure on the balloon is 0.350 atm. What is the new volume of the balloon at this height? (assume constant temp.) 4. A gas with a pressure of 875 mm Hg at 305C occupies 2.50 L. What volume will the gas occupy at STP? 5. What volume will 5.70 mol of nitrogen gas, N2, occupy at 54.0C and 675 mm Hg? 6. A 54.76 g sample of carbon dioxide gas is sealed in a 7.55 L container and heated to a temperature of 354C. What is the pressure of the gas? 7. Calculate the mass of oxygen gas, O2, measured at 1765 mm Hg and 35.5C in 4.35 L. 8. How many grams of helium, He, will be required to fill 12 balloons at STP, assuming that each balloon has a volume of 455 mL? 9. What is the molecular weight of a gas that occupies a volume of 278 mL, at a pressure of 680.0 mm Hg, a temperature of 21.7C, and a mass of 35.9 g? 10. What is the molecular weight of an ideal gas if 0.520 g of the gas occupies 610.0 mL at 385 mm Hg and 45.0C? 11. Analysis of a gaseous compound shows that it contains 85.7% carbon and 14.3% hydrogen by mass. At standard conditions, 100.0 mL of the compound has a mass of 0.188 gram. What is its molecular formula? 12. Calculate the density of NO2 gas, in g/L, at 1.24 atm and 50.0C? 13. A 300.0 mL sample of hydrogen was collected over water 21.0C on a day when the atmospheric pressure was 748 mm Hg. a) What is the partial pressure of the hydrogen gas? b) How many moles of H2 were present? c) What would be the mass of the hydrogen sample if it were dry?