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Dalton’s Law of Partial Pressures

Dalton’s Law of Partial Pressures. What happens when you put two gases in the same space?. Dalton’s Law of Partial Pressure. When two gases occupy the same space, the total pressure is the sum of the pressures exerted by each gas Each gas exerts pressure as if the other was not present.

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Dalton’s Law of Partial Pressures

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  1. Dalton’s Law of Partial Pressures What happens when you put two gases in the same space?

  2. Dalton’s Law of Partial Pressure • When two gases occupy the same space, the total pressure is the sum of the pressures exerted by each gas • Each gas exerts pressure as if the other was not present

  3. Partial Pressure • Oxygen exerts a certain amount of pressure, based on temperature, volume, & number of particles • Nitrogen exerts a certain amount of pressure, based on temperature, volume, & number of particles

  4. Partial Pressure • When combined, the total pressure is the sum of the individual pressures

  5. Dalton’s Law of Partial Pressures • For a mixture of gases (labeled a, b, & c) • Ptotal = Pa + Pb + Pc

  6. Water Displacement Gas Collection • As discussed earlier, one of the methods to determine the volume of a gas collected is to measure the amount of water displaced from a flask • You performed a procedure similar to this during the Charles’s Law Lab • When this is done, the gas present in the collection vessel contains the desired gas and water vapor

  7. Water Displacement Gas Collection

  8. Vapor Pressure of Water

  9. Dalton’s Law is useful when gases are collected over water • To find the pressure of the gas collected, the water vapor pressure must be subtracted from the total pressure • Example: • 98.2 kPa total pressure in collection flask at 20oC • 17.5 kPa water vapor pressure at 20oC • 98.2 kPa – 17.5 kPa = 80.7 kPa of gas

  10. Mole Fractions & Partial Pressure • The mole fraction of any gas can be determined by dividing the moles of gas A by the total moles of gas in the container • Mole Fraction of A = (moles A)/(total moles) • Mole fraction should always be less than 1 • Mole fraction can be multiplied by the total pressure to obtain the partial pressure for a gas

  11. Mole Fractions & Partial Pressure • What is the partial pressure of oxygen in a sample of air with a total pressure of 738 torr? • % oxygen in air ≈ 21% • PO2 = (0.21)(738 torr) = 155 torr

  12. Oxygen Partial Pressure • Partial pressure of O2 must remain above 14 kPa to be absorbed efficiently by your body • O2 makes up about 21% of the number of particles in the air and the total pressure of the air at sea level is 101.325 kPa • As the altitude increases, the mole fraction of O2 remains the same, but the total pressure decreases • This reduces the partial pressure of O2 to dangerously low levels in some instances

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