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Chapter 12 Molecular Composition of Gases

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Molar Volume of a Gas One mole of a gas has the same volume at STP as any other gas 22.4 L / mole at STP

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What volume would mol of oxygen gas occupy at STP? What about mol of nitrogen gas at STP? Practice Problem

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Ideal Gas Law PV = nRT n = Number of Moles R = Ideal Gas Constant L. atm/ mol. K V = Volume (must be in liters) P = Pressure (must be in atmospheres) T = Temperature (must be in Kelvin)

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Practice Problem What is the P (in atm) exerted by a mol sample of nitrogen gas in a 10.0 L container at 298 K? PV = nRT rearranges to P= nRT/ V (0.500 mol)( L. atm/mol. K)(298K) 10.0 L Pressure = 1.22 atm

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Gas Stoichiometry Uses volume-volume calculations (e.g. L L) Liters can be used just like mole to mole ratios in a factor label problem

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Practice Problem How many L of oxygen are required for the complete combustion of L of propane? C 3 H 8 (g) + 5O 2 (g) 3CO 2 (g) + 4H 2 O(g) Ratio: 5 L O 2 makes 1 L C 3 H L C 3 H 8 x 5 L O 2 1 L C 3 H 8 = 1.25 L O 2

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Practice Problem How many grams of calcium carbonate must be decomposed to produce 2.00 L of CO 2 at STP? CaCO 3 (s) CaO(s) + CO 2 (g) 2.00 L CO 2 x 1 mol CO 2 x 1 mol CaCO L CO 2 1 mol CO 2 x g CaCO 3 1 mol CaCO g CaCO 3

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Grahams Law

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Grahams Law of Effusion or Diffusion The rate of effusion or diffusion is inversely proportional to the square roots of their molar masses Rate A = M B Rate B M A

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Practice Problem Compare the rate of effusion of hydrogen and nitrogen gas rate H 2 = 28 = 14 = 3.7 rate N 2 2 Hydrogen effuses 3.7 times faster than nitrogen

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