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Day 5 – Notes. Unit: Gas Laws. Dalton’s Law of Partial Pressures, Grahams Law, and Real vs. Ideal Gases. After today you will be able to…. Describe Dalton’s law of partial pressures and calculate P total or a partial pressure

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unit gas laws

Day 5– Notes

Unit: Gas Laws

Dalton’s Law of Partial Pressures, Grahams Law,

and Real vs. Ideal Gases

after today you will be able to
After today you will be able to…
  • Describe Dalton’s law of partial pressures and calculate Ptotalor a partial pressure
  • Explain Graham’s law of effusion and calculate the rate at which gases effuse
  • Explain what is meant by the term “real” vs. “ideal” gases

Recall, gas pressure results from collisions of gas particles.

  • Gas pressure depends on the amount of gas and the KE of its particles.
  • Since particles in a mixture of gases at the same temperature contain the same average KE, the kind of particle is unimportant.

“The total pressure of a mixture of gases is equal to the sum of the individual (partial) pressures.”

dalton s law of partial pressures
Dalton’s Law of Partial Pressures

Units of pressure must match!

Ptotal= P1 + P2 + P3…

example dalton s law
Example: Dalton’s Law

What is the total pressure for a mixture of O2 and CO2 if PO2= 0.719 atm and PCO2= 423mmHg.

PO2= 0.719atm







Ptotal=546mmHg + 423mmHg


thomas graham 1846
Thomas Graham (1846)
  • Diffusion: Is the tendency of gas particles to spontaneously spread out until uniformly distributed.
  • Effusion: The escape of a gas through a tiny pinhole in a container of gas.
    • Gases with lower molar masses effuse more quickly.

“The rate of effusion of a gas is inversely proportional to the square root of the gas’s molar mass.”

graham s law of effusion
Graham’s Law of Effusion

Always place the larger molar mass in the numerator!

Rate A√MMB

Rate B √MMA


example graham s law
Example: Graham’s Law

Which gas effuses faster, H2 or Cl2? How much faster?

Rate H2√MMCl2

Rate Cl2√MMH2

Rate H2√(70.90)

Rate Cl2√(2.02)



H2 effuses 5.92x faster than Cl2

= 5.92x

real vs ideal gases
Real vs. Ideal Gases
  • The gas laws we’ve learned in this unit are based on a gas that behaves “ideally.”
  • An ideal gas has:
    • No molecular volume
    • No attractive forces
  • In reality, there are no perfectly ideal gases. But, under most conditions, real gases will approximate ideal gas behavior.
  • However, under certain conditions, real gases will deviate from ideal gas behavior.
real vs ideal gases1
Real vs. Ideal Gases
  • These deviations occur for:
  • High pressure: Gas particles are pushed closer together, more attractive forces result.
  • Low Temperature: The gas is compressed, there are more attractive forces.
  • High molar mass: Higher molar mass of the molecule usually means larger volume.
  • Polar molecules: Unequal sharing of electrons creates an attraction between molecules.