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**Molecular Composition of Gases**

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

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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 = 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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**C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g)**

Practice Problem How many L of oxygen are required for the complete combustion of L of propane? C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g) Ratio: 5 L O2 makes 1 L C3H8 0.250 L C3H8 x 5 L O2 1 L C3H8 = 1.25 L O2

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**CaCO3(s) CaO(s) + CO2(g)**

Practice Problem How many grams of calcium carbonate must be decomposed to produce 2.00 L of CO2 at STP? CaCO3(s) CaO(s) + CO2(g) 2.00 L CO2 x 1 mol CO2 x 1 mol CaCO3 22.4 L CO mol CO2 x g CaCO3 1 mol CaCO3 8.94 g CaCO3

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Graham’s Law Diffusion Effusion

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**Graham’s Law of Effusion or Diffusion**

The rate of effusion or diffusion is inversely proportional to the square roots of their molar masses Rate A = MB Rate B MA

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Practice Problem Compare the rate of effusion of hydrogen and nitrogen gas rate H2 = = = 3.7 rate N Hydrogen effuses 3.7 times faster than nitrogen

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