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### Unit: Gas Laws

### Questions? to the square root of the gas’s molar mass.”Complete WS 5

Day 5– Notes

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 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.”

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

PCO2=423mmHg

760mmHg

x

=

546mmHg

1atm

Ptotal=546mmHg + 423mmHg

Ptotal=969mmHg

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 to the square root of the gas’s molar mass.”

Always place the larger molar mass in the numerator!

Rate A√MMB

Rate B √MMA

=

Example: Graham’s Law to the square root of the gas’s molar mass.”

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 to the square root of the gas’s molar mass.”

- 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 Gases to the square root of the gas’s molar mass.”

- 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.

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