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Dalton ’ s Law

Dalton ’ s Law. Mixtures of Gases. Introduction. From the kinetic theory of gases, at a given temperature and in a given volume gas pressure depends only on the number of atoms colliding with the walls of the container the more collisions, the higher the pressure

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Dalton ’ s Law

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  1. Dalton’s Law • Mixtures of Gases

  2. Introduction • From the kinetic theory of gases, at a given temperature and in a given volume • gas pressure depends only on the number of atoms colliding with the walls of the container • the more collisions, the higher the pressure • the fewer the collisions, the lower the pressure • Therefore, the larger the amount of gas in a container, the higher the pressure.

  3. Introduction • If we have a mixture of gases in our container • each different set of gas particles will contribute its own set of collisions • the identity of the individual gases is irrelevant. • This means that each gas will have its own pressure. • This pressure is called the partial pressure of the gas.

  4. Introduction • This was studied by Dalton who proposed the following law (Dalton’s Law of Partial Pressures): • “In a mixture of gases, the total pressure is the sum of the partial pressures of the gases.” • We use the equation: • Ptotal = P1 + P2 + P3 + ...

  5. Application • For example,In dry air we have: • 78.09% N2 • 20.95% O2 • The partial pressure of N2 is - • (0.7808)(101.3 kPa) = 79.11 kPa • The partial pressure of O2 is - • (0.2095)(101.3 kPa) = 21.22 kPa

  6. Application • For dry air in general:

  7. Example 1 A gas mixture containing oxygen, nitrogen, and carbon dioxide has PO2 = 20.1 kPa, PN2 = 18.3 kPa, and PCO2 = 34.4 kPa. What is Ptotal? • Ptotal = P1 + P2 + P3 • Ptotal = PO2 + PN2 + PCO2 • Ptotal = 20.1 kPa + 18.3 kPa + 34.4 kPa • Ptotal = 72.8 kPa

  8. Example 2 A gas mixture containing oxygen, nitrogen, and argon has a total pressure of 50.2 kPa. If PO2 = 20.1 kPa and PN2 = 18.3 kPa what is PAr? • Ptotal = P1 + P2 + P3 • Ptotal = PO2 + PN2 + PAr • PAr = Ptotal - PO2 - PN2 • PAr = 50.2 kPa - 20.1 kPa - 18.3 kPa • Ptotal = 11.8 kPa

  9. Water Vapor Pressure • Most often in chemistry we use Dalton’s law of partial pressure when we collect gas over water. • When we generate a gas in a chemical reaction, we often want to capture that gas. • Usually, we bubble the gas from the reaction into a water filled collection tube.

  10. Water Vapor Pressure • We can measure the volume of the tube directly. • And (after some adjustment), we can assume the pressure in the collection tube is the same as atmospheric pressure. • But, the gas in the tube has the gas we want andwater vapor.

  11. Water Vapor Pressure • We can use Dalton’s law of partial pressures to subtract out the water vapor so we know just the pressure of the gas we collected. • We use a water vapor pressure data table to determine the partial pressure of water at any given temperature.

  12. Water Vapor Pressure • A typical water vapor pressure table looks like this:

  13. Water Vapor Pressure • Patmosphere = Pwater + Pgas • Pgas = Patmosphere - Pwater

  14. Example 3 Hydrogen gas is collected over water at a temperature of 23.0°C with an atmospheric pressure of 754.2 mm Hg. What is the partial pressure of the hydrogen gas in the collection tube. • Patmosphere = 754.2 mm Hg • Pwater = 21.1 mm Hg (from table) • Patmosphere = Phydrogen + Pwater • Phydrogen = Patmosphere - Pwater • Phydrogen = 754.2 mm Hg - 21.1 mm Hg • Ptotal = 733.1 mm Hg

  15. Summary • Dalton’s Law of Partial Pressures: • “In a mixture of gases, the total pressure is the sum of the partial pressures of the gases.” • We use the equation: • Ptotal = P1 + P2 + P3 + ...

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