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Ideal Gases

Ideal Gases. Ideal Gases. Now that we know how gases behave when we manipulate P, V, and T, it’s time to start thinking about how to deal with things like moles and grams . After all, if we’re going to do chemical reactions with gases, we’ll need to know how to calculate these!.

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Ideal Gases

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  1. Ideal Gases

  2. Ideal Gases • Now that we know how gases behave when we manipulate P, V, and T, it’s time to start thinking about how to deal with things like moles and grams. • After all, if we’re going to do chemical reactions with gases, we’ll need to know how to calculate these!

  3. Avogadro’s law • One mole of every gas has the same volume. • This law assumes that all gases behave perfectly and identically according to the rules of the kinetic molecular theory. Though not precisely true, it gives us very good answers under most conditions.

  4. Ideal gas • A gas that behaves according to the kinetic molecular theory.

  5. Properties of an ideal gas • No intermolecular forces, infinitely small, etc. • There is no ideal gas in the real world, but some gases come closer than others: • The gas molecules are small. • The gas molecules have very weak intermolecular forces. • The gas molecules are very hot, so they move quickly around and don’t interact with each other much. • The gas is at low pressure, so the molecules have a lot of space between them.

  6. Ideal Gases • Assuming that all gases are ideal, we can use an equation to relate the number of moles to the pressure (P), volume (V), and temperature (T), giving us the… • IDEAL GAS LAW!!!!

  7. PV = nRT • P = pressure (in atm of kPa) • V = volume (L) • n = number of moles • T = temperature (Kelvin) • R = ideal gas constant (depends on the unit of pressure used) • 8.314 L kPa/mol K • 0.08206 L atm/mol K

  8. Examples: • If I have 10 liters of a gas at a pressure of 1.5 atm and a temperature of 250 C, how many moles of gas do I have? • 0.61 mol. • If I have 3.5 moles of a gas at a pressure of 895 kPa and they take up a volume of 50 L, what’s the temperature? • 1538 K

  9. Dalton’s Law of Partial Pressures • So far, we’ve assumed that all the gases we’ve been working with are pure. However, this isn’t true most of the time. • Most gases we use are actually mixtures of several different elements or compounds.

  10. Total pressure = sum of P for all gases • Partial pressure: The pressure that each of the components in a mixture of gases would exert if the other gases in the mixture were removed. • Dalton’s Law: Ptot = P1 + P2 + P3 + …

  11. Examples: • A container contains three gases. If the partial pressure of carbon dioxide is 0.50 atm, the partial pressure of nitrogen is 0.30 atm, and the partial pressure of methane is 0.40 atm, what’s the total pressure inside the container? Ptot = 0.50 atm + 0.30 atm + 0.40 atm = 1.20 atm • A container has a total internal pressure of 1.50 atm. This container contains two gases, nitrogen (which has a partial pressure of 0.80 atm) and oxygen (with unknown partial pressure). Given this information, what’s the partial pressure of oxygen in the container? Ptot = Pnitrogen + Poxygen 1.50 atm = 0.80 atm + X X = 0.70 atm

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