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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

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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

Unit gas laws

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.

Example composition of dry air

Example: Composition of Dry Air

Unit gas laws

“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.

Unit gas laws

“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.

Questions complete ws 5

Questions?Complete WS 5

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