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lecture20.updated - Lecture 20: Gases: wrap up Lecture 20:...

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Lecture 20: Gases: wrap up Combined Gas Law: 2-state problems P 1 V 1 n 1 T 1 = R P 2 V 2 n 2 T 2 = If n remains constant (same system): P 1 V 1 T 1 P 2 V 2 T 2 = combined gas law Always use T in Kelvins
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P 1 V 1 T 1 P 2 V 2 T 2 = Combined Gas Law A gas occupies 401 mL at P = 1.000 atm. What will be its volume if P is decreased to 0.750 atm? T is held constant. T 1 = T 2 (cancel out) P 1 V 1 P 2 V 2 = 1.000 atm (401 mL) = (0.750 atm) V 2 V 2 = 401 mL (1.000 atm/ 0.750 atm) = 535 mL Gas Density and Molar Masses For a gas with mass, m, and molecular mass, M: PV = nRT = RT m M d is proportional to M . M (g/mol) d ( g / L) He 4.003 0.179 air 28.8* 1.28 O 2 31.999 1.428 SF 6 146.06 6.516 STP densities; *Average mass. m V PM RT d = =
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A flask contains 1.00 L of a gas at 0.850 atm and 20°C. The mass of the gas is 1.13 g. What is the molar mass of the gas? What is its identity? Calculate n gas . n = PV = (0.850 atm)(1.00 L) = 0.03533 mol RT (0.08206 L atm mol -1 K -1 )(293.2 K) Molar mass = 1.13 g 0.03533 mol = 32.0 g/mol ( O 2 , SiH 4 , N 2 H 4 , …) Molar Mass Dalton Dalton ’s law of partial pressures s law of partial pressures “The total pressure of mixture of gases is the sum of the partial pressure of the individual gases in the mixture.” P total = P 1 + P 2 + P 3 + … Gas Mixtures & Partial Pressures
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Gas Mixtures & Partial Pressures Gas Mixtures & Partial Pressures 1. Each gas obeys it’s own ideal gas law. 2. Combined, the gases obey the ideal gas law. This is because the identity of the gas doesn’t matter, only the number of molecules.
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Mole Fractions and the Ideal Gas Law P total = P 1 + P 2 + … (P 1 = n 1 RT / V) = n 1 RT / V + n 2 RT / V + . .. = (n 1 + n 2 + . ..) RT / V P total = n total RT / V So P 1 / P total = n 1 /n total = X 1 P 2 / P total = n 2 /n total = X 2 etc. with X 1 = mole fraction mole fraction of gas 1 = ( n 1 / n total ) etc. Notice that: X 1 + X 2 + X 3 + …. . = 1 Life and partial pressure of oxygen
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Life and partial pressure of oxygen normal low P O2 blockage (pneumonia) Life and partial pressure of oxygen smoker (CO binds 250 times more strongly than O 2 )
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A sample of gas is prepared by combining 4.2 g O 2 (MM=32) and 7.5g CO 2 (MM=44). The total pressure is 0.75 atm. What is the partial pressure of O 2 ?
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This note was uploaded on 11/18/2011 for the course CHE 131 taught by Professor Kerber during the Spring '08 term at SUNY Stony Brook.

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lecture20.updated - Lecture 20: Gases: wrap up Lecture 20:...

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