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Review Chapter 11 Numbers and formulae to know: Gases occupy 22.4L/mol at STP STP is 273 K and 1 atm 1 atm = 760 mmHg = 760 Torr = 101 kPa K = °C + 273 The Ideal Gas Law: PV = nRT Boyle’s Law P1V1 = P2V2 Charles’ Law V1 = V2 T1 T2 Gay-Lussac’s Law P1 = P2 T1 T2 Avogadro’s Law V1 = V2 n1 n2 The Combined Gas Law P1V1 = P2V2 T1 T2 The Universal Gas Constant R = 0.0821 L×atm K×mol n.b. you must convert to liters, atmospheres, Kelvins and moles to use R. Concepts: An ideal gas, according to kinetic theory, is one with very tiny molecules that travel quickly in straight lines; when the molecules collide they experience perfectly elastic collisions; the kinetic energy of the molecules is directly proportional to their Kelvin temperature. A real gas is one whose behavior is affected by its particles’ volume, mass and attractive forces, so the particles do not experience perfectly elastic collisions and therefore do not follow the ideal gas law PV = nRT. Vapor pressure is the pressure exerted by molecules in the vapor above a liquid – it increases with increasing temperature. Dalton’s Law of Partial Pressures states that the partial pressures exerted by each gas in a mixture can be added to find the total pressure: Ptotal = P1 + P2 + P3 …. Px Practice problems: 1. If pressure decreases, volume ___increases______. If temperature increases, pressure ____increases______. If number of moles increases, pressure ___increases_______. If temperature decreases, volume ____decreases________. 2. A gas occupying 725mL at a pressure of 97.0 kPa is allowed to expand until its pressure becomes 54.1 kPa. What is its final volume? Boyle’s Law V2 = 1.30 x 103mL 3. A sample of nitrogen gas kept in a container of volume 2.3L and at a temperature of 290 K exerts a pressure of 595mmHg. Calculate the number of moles of gas present. Ideal Gas Law (.783 atm)(2.3 L) = n (0.0821)(290 K) n = 0.076 mol 4. A 2.5L flask at 15ºC contains a mixture of three gases, N2 at 0.32 atm, He at 0.15 atm and Ne at 0.42 atm. Calculate the total pressure of the mixture. Dalton’s Law 0.32 + 0.15 + 0.42 = 0.89 atm 5. A certain quantity of gas at 25ºC and a pressure of 0.800 atm is contained in a glass vessel that can withstand a pressure of 6.5 atm. How much can you increase the temperature of the gas without bursting the vessel? Gay-Lussac’s Law T2 = 2400 K so the temperature could be increased by 2100 K without bursting the vessel (2 sig figs) 6. A balloon has a volume of 68 L and contains 1.98 mol of nitrogen gas. If an additional 0.25 moles of gas are blown into the balloon, what will its new volume be? (note that n2 = 1.98 + 0.25) Avogadro’s Law V2 = 77 L 7. The temperature of 34.7 L of methane gas is increased by a factor of two. What is the resulting volume of the gas? Charles’ Law V = 69.4 L 8. A gas-filled balloon having a volume of 2.5 L at 1.2 atm and 25ºC is allowed to rise to the stratosphere, where the temperature and pressure are -23ºC and 3.0 x 10-3 atm, respectively. Calculate the final volume of the balloon. Combined Gas Law V2 = 840 L 9. A student creates hydrogen gas over water via the following reaction: Mg(s) + 2HNO3(aq) H2(g) + Mg(NO3)2(aq) If he used 1.00 g of Mg, the temperature in the lab was 25ºC and the atmospheric pressure was 771 mmHg, what volume of H2 gas should he have made? (The vapor pressure of water at 25ºC is 23.8 mmHg.) Ideal Gas Law 1.00 g x 1 mol/24.3g = 0.0412 moles 771 mmHg – 23.8 mmHg = 747 mmHg 747 mmHg x 1 atm/760 mmHg = 0.983 atm 25 ºC + 273 = 298 K (0.983 atm)(V) = (0.0412 mol)(0.0821)(298 K) V = 1.05 L

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posted: | 11/15/2012 |

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