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Unit: Gas Laws

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Day 5– Notes

Unit: Gas Laws

Dalton’s Law of Partial Pressures, Grahams Law,

and Real vs. Ideal Gases

- 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.”

Units of pressure must match!

Ptotal= P1 + P2 + P3…

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

- 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.”

Always place the larger molar mass in the numerator!

Rate A√MMB

Rate B √MMA

=

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

- 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.

- 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