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Understanding Dalton's Law of Partial Pressures and the Behavior of Wet and Dry Gases

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This review covers Dalton's Law of Partial Pressures and the properties of gases, including pressure, volume, and temperature relationships. It highlights the ideal gas equation (PV=nRT) and provides calculations for changes in gases during mixtures. Additionally, it explains how to handle gases collected over water, which contain water vapor that contributes to total pressure. Key principles, such as determining dry gas pressure from total pressure and water vapor pressure, are illustrated through practical examples.

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Understanding Dalton's Law of Partial Pressures and the Behavior of Wet and Dry Gases

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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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