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Dalton’s Law

Dalton’s Law. P total = P 1 + P 2 + P 3 +. Dalton’s Law of Partial Pressure. The total pressure of a mixture of non-reacting gases is equal to the sum of the partial pressures of the individual gases (P total = P 1 + P 2 + P 3 + …). Dalton’s Law.

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Dalton’s Law

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  1. Dalton’s Law Ptotal = P1 + P2 + P3 + . . .

  2. Dalton’s Law of Partial Pressure The total pressure of a mixture of non-reacting gases is equal to the sum of the partial pressures of the individual gases (Ptotal = P1 + P2 + P3 + …)

  3. Dalton’s Law For example: If we have a flask containing nitrogen gas, whose partial pressure is .78 atm and oxygen gas, whose partial pressure is .20 atm, the total pressure in the flask is Ptotal = PN2 + PO2 Ptotal = .78 + .20 = .98 atm

  4. Dalton’s Law Another way to think about this is through the use of the mole fraction. The mole fraction is rather like calculating the percent of each component.

  5. Dalton’s Law The mole fraction is written as follows XN2=moles N2 total moles

  6. Dalton’s Law We can rearrange Dalton’s Law to use the mole fraction as follows: Partial Pressure of a Gas= Mole Fraction of a Gasx the Total Pressure Pgas = Xgas (Ptotal) Or specifically: PN2 = XN2 (Ptotal)

  7. Example • What is the partial pressure of carbon dioxide in a container that holds 5.0 moles of CO2, 3.0 moles of N2, and 1.0 mole of H2 and has a total pressure of 1.05 atm? • Step One: find the mole fraction of CO2 5.0 moles CO2/ 9.0 moles total = 0.56 • Step Two: multiply the mole fraction by the total pressure 0.56 x 1.05 atm = 0.58atm

  8. Dalton’s Law Another example: ordinary air contains 78.084% nitrogen. What is the partial pressure of nitrogen in the atmosphere if atmospheric pressure is 640 mmHg.

  9. Dalton’s Law The mole fraction of N2 must be .78084, so PN2 = .78084(Ptotal) If the atmospheric pressure is 640 mmHg, PN2 = .78084(640) = 500 mmHg

  10. Dalton’s Law • Two flasks are connected with a stopcock. The first flask has a volume of 6.0 L and contains Nitrogen gas at a pressure of 0.75 atm. The second flask has a volume of 9.0 L and contains Oxygen gas at a pressure of 1.25 atm. When the stopcock is opened, the gases are free to mix. What will the pressure of the resulting mixture of gases be? • Hint: Find the new pressure of each individual gas first. Then, add them together.

  11. 1.05 atm

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