VIEWS: 40 PAGES: 51 POSTED ON: 11/25/2011
Chapter 11 GASES If force is held constant as surface area decreases, pressure a.remains constant. b.decreases. c.increases. d.increases or decreases, depending on the volume change. Why does a can collapse when a vacuum pump removes air from the can? a.The inside and outside forces balance out and crush the can. b.The unbalanced outside force from atmospheric pressure crushes the can. c.The atmosphere exerts pressure on the inside of the can and crushes it. d.The vacuum pump creates a force that crushes the can. If the height of mercury in a barometer at 0ºC is less than 760 mm Hg, then a.the atmospheric pressure is less than standard atmospheric pressure. b.the atmospheric pressure is greater than standard atmospheric pressure. c.the atmospheric pressure is equal to standard atmospheric pressure. d.the atmospheric pressure cannot be determined. Convert the pressure 0.840 atm to mm Hg. a.365 mm Hg b.437 mm Hg c.638 mm Hg d.780 mm Hg Convert the pressure 1.30 atm to kPa. a.2 kPa b.115 kPa c.132 kPa d.245 kPa Standard temperature is exactly a.100ºC. b.273ºC. c.0ºC. d.0 K. To correct for the partial pressure of water vapor in a gas collection bottle, the vapor pressure of H2O at the collecting temperature is generally a.subtracted from the partial pressure of the collected gas. b.added to the pressure of the collected gas. c.subtracted from the atmospheric pressure. d.added to the atmospheric pressure. Three samples of gas each exert 740. mm Hg in separate 2 L containers. What pressure do they exert if they are all placed in a single 2 L container? a.247 mm Hg b.740 mm Hg c.1.48 x 103 mm Hg d.2.22 x 103 mm Hg The volume of a gas is 400.0 mL when the pressure is 1.00 atm. At the same temperature, what is the pressure at which the volume of the gas is 2.0 L? a.0.5 atm b.5.0 atm c.0.20 atm d.800 atm If the temperature of a fixed quantity of gas decreases and the pressure remains unchanged, a.its volume increases. b.its volume is unchanged. c.its volume decreases. d.its density decreases. The volume of a gas is 5.0 L when the temperature is 5.0ºC. If the temperature is increased to 10.0ºC without changing the pressure, what is the new volume? a.2.5 L b.4.8 L c.5.1 L d.10.0 L Why could the pressure of a sample of gas at a constant volume fall 75 mm Hg? a.The container exploded. b.The temperature increased. c.The temperature decreased. d.The volume increased. Why does the air pressure inside the tires of a car increase when the car is driven? a.Some of the air has leaked out. b.The air particles collide with the tire after the car is in motion. c.The air particles inside the tire increase their speed because their temperature rises. d.The atmosphere compresses the tire. On a cold winter morning when the temperature is – 13ºC, the air pressure in an automobile tire is 1.5 atm. If the volume does not change, what is the pressure after the tire has warmed to 15ºC? a.–1.5 atm b.1.7 atm c.3.0 atm d.19.5 atm The volume of a gas collected when the temperature is 11.0ºC and the pressure is 710 mm Hg measures 14.8 mL. What is the calculated volume of the gas at 20.0ºC and 740 mm Hg? a.7.8 mL b.13.7 mL c.14.6 mL d.15 mL Gas Volumes Gay-Lussac’s Law of Combining Volumes of Gases At constant temperature and pressure, the volumes of gaseous reactants and products can be expresses as ratios of small whole numbers hydrogen gas + oxygen gas water vapor 2 L (2 volumes) 1 L (1 volume) 2 L (2 volumes) Avogadro’s Law Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules The equation for this relationship is shown below, where V is the volume, k is a constant, and n is the amount of moles of the gas. V = kn Avogadro’s Law Dalton had guessed that the formula for water was HO, but Avogadro’s reasoning established that water must contain twice as many H atoms as O atoms because of the volume ratios in which the gases combine: hydrogen gas + oxygen gas water vapor 2 L (2 volumes) 1 L (1 volume) 2 L (2 volumes) Gay-Lussac recognized that at constant temperature and pressure, the volumes of gaseous reactants and products a.always equal 1 L. b.add up to 22.4 L. c.equal R. d.can be expressed as ratios of small whole numbers. The law of combining volumes applies only to gas volumes a.measured at constant temperature and pressure. b.that equal 1 L. c.that equal 22.4 L. d.measured at STP. If 0.5 L of O2(g) reacts with H2 to produce 1 L of H2O(g), what is the volume of H2O(g) obtained from 1 L of O2(g)? a.0.5 L b.1.5 L c.2 L d.2.5 L Molar Volume of a Gas Recall that one mole of a substance contains a number of particles equal to Avogadro’s constant (6.022 1023). According to Avogadro’s law, one mole of any gas will occupy the same volume as one mole of any other gas at the same conditions, despite mass differences. Molar Volume of a Gas The volume occupied by one mole of gas at STP is known as the standard molar volume of a gas which is 22.414 10 L (rounded to 22.4 L). Gas Stoichiometry The coefficients in chemical equations of gas reactions reflect not only molar ratios, but also volume ratios (assuming conditions remain the same). Example-reaction of carbon dioxide formation: 2CO(g) + O2(g) 2CO2(g) In the reaction represented by the equation N2(g) + 2O2(g) -> 2NO2(g), what is the volume ratio of N2 to NO2? a.1:1 b.1:2 c.2:1 d.2:5 The equation for the production of methane is C + 2H2(g) -> CH4(g). How many liters of hydrogen are needed to produce 20. L of methane? a.2.0 L b.20. L c.22.4 L d.40. L What is the number of moles of H2 produced when 23 g of sodium react with water according to the equation 2Na(s) + 2H2O(l) -> 2NaOH(aq) + H2(g)? a.0.50 mol b.1.0 mol c.2.0 mol d.4.0 mol If the temperature of a container of gas remains constant, how could the pressure of the gas increase? a.The mass of the gas molecules increase. b.The diffusion of the gas molecules increases. c.The size of the container increases. d.The number of gas molecules in the container increases. If gas A has a molar mass greater than that of gas B and samples of each gas at identical temperatures and pressures contain equal numbers of molecules, then a.the volumes of gas A and gas B are equal. b.the volume of gas A is greater than that of gas B. c.the volume of gas B is greater than that of gas A. d.their volumes are proportional to their molar masses. At STP, the standard molar volume of a gas of known volume can be used to calculate the a.number of moles of gas. b.rate of diffusion. c.gram-molecular weight. d.gram-molecular volume. Ideal Gas Law It is stated as shown below, where R is a constant: PV = nRT In the equation representing the ideal gas law, the constant R is known as the ideal gas constant. Its value depends on the units chosen for pressure, volume, and temperature in the rest of the equation. Ideal Gas Constants Sample Problem I What is the pressure in atmospheres exerted by a 0.500 mol sample of nitrogen gas in a 10.0 L container at 298 K? Sample Problem I The ideal gas law is equivalent to Boyle's law when a.the number of moles and the pressure are constant. b.R equals zero. c.the pressure is 1 atm. d.the number of moles and the temperature are constant. A 1.00 L sample of a gas has a mass of 1.92 g at STP. What is the molar mass of the gas? a.1.92 g/mol b.19.2 g/mol c.22.4 g/mol d.43.0 g/mol Iron(IV) oxide, FeO2, is produced by the reaction Fe + O2 ® FeO2 (87.8 g/mol). How many grams of FeO2 can be produced from 50.0 L of O2 at STP? a.19.5 g b.37.8 g c.50 g d.196. g Calculate the approximate volume of a 0.600 mol sample of gas at 15.0°C and a pressure of 1.10 atm. a.12.9 L b.22.4 L c.24.6 L d.139 L What is the pressure exerted by 1.2 mol of a gas with a temperature of 20.ºC and a volume of 9.5 L? a.0.030 atm b.1.0 atm c.3.0 atm d.30. atm Diffusion and Effusion The gradual mixing of two or more gases due to their spontaneous, random motion is known as diffusion. Effusion is the process whereby the molecules of a gas confined in a container randomly pass through a tiny opening in the container. Graham’s Law of Effusion Rates of effusion and diffusion depend on the relative velocities of gas molecules. The velocity of a gas varies inversely with the square root of its molar mass. Recall that the average kinetic energy of the molecules in any gas depends only the temperature and equals Graham’s Law of Effusion Graham’s Law Sample Problem Compare the rates of effusion of hydrogen and oxygen at the same temperature and pressure. Sample Problem Hydrogen effuses 3.98 times faster than oxygen. If a gas with an odor is released in a room, it can quickly be detected across the room because it a.diffuses. b.is dense. c.is compressed. d.condenses. What is the process by which molecules of a gas randomly encounter and pass through a small opening in a container? a.diffusion b.vaporization c.distillation d.effusion According to Graham's law, two gases at the same temperature and pressure will have different rates of effusion because they have different a.volumes. b.molar masses. c.kinetic energies. d.condensation points. How many times greater is the rate of effusion of molecular fluorine than that of molecular bromine at the same temperature and pressure? a.2.051 b.3.062 c.4.450 d.7.280 A sample of hydrogen gas diffuses 3.8 times faster than an unknown gas diffuses. What is the molar mass of the unknown gas? a.4.0 g/mol b.7.6 g/mol c.22 g/mol d.29 g/mol