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Dalton’s Law of Partial Pressures

Dalton’s Law of Partial Pressures. Plus “wet” and “dry” gases. Review. The properties of a gas (pressure, volume, and temperature) are all related. PV = nRT for an ideal gas P 1 V 1 /T 1 = P 2 V 2 /T 2 Used to calculate changes in gases. Mixtures of Gases.

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Dalton’s Law of Partial Pressures

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  1. Dalton’s Law of Partial Pressures Plus “wet” and “dry” gases

  2. Review • The properties of a gas (pressure, volume, and temperature) are all related. • PV = nRT for an ideal gas • P1V1/T1 = P2V2/T2 • Used to calculate changes in gases.

  3. Mixtures of Gases • In a non-reacting mixture of gases, the total pressure = the sum of the pressures of each gas. • Ptotal = PA + PB + PC + ... • A mixture of He, Ne, and Ar has a total pressure of 900 mmHg. If PHe = 200 mmHg and PNe = 600 mmHg, what is PAr? • Ptotal = PHe + PNe + PAr • 900 mmHg = 200 mmHg + 600 mmHg + PAr • 900 mmHg = 800 mmHg + PAr • PAr = 100 mmHg

  4. “Wet” and “Dry” Gases • In the laboratory, many gases are collected over water. • Whenever a gas is collected over water, it contains water vapor as well. • The water vapor contributes some pressure to the gas mixture. • In order to work with the captured gas, you must account for the vapor pressure of the water.

  5. “Wet” and “Dry” Gases • Water vapor pressure depends on temperature. • Tables of water vapor pressure at various temperatures are widely available. • No need to memorize these numbers. • When a gas is collected over water: • Ptotal = Pgas + Pwater vapor

  6. Water Vapor Pressure

  7. Dalton’s Law of Partial Pressures • A sample of butane gas is collected over water at 25º C. The vapor pressure of water at 25º C is 23.76 mmHg. If the total pressure in the bottle is 772.33 mmHg, what is the pressure of the dry gas? • Ptotal = Pgas + Pwater vapor • 772.33 mmHg = Pgas + 23.76 mmHg • Pgas = 748.57 mmHg

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