The Behavior of Gases Part 2. Ideal Gases Ideal Gas Law: Ideal Gas Law: The relationship PV = nRT, which describes the behavior of ideal gases. The relationship

The Behavior of GasesThe Behavior of Gases

Part 2Part 2

Ideal GasesIdeal Gases

Ideal Gas Law:Ideal Gas Law: The relationship PV = nRT, which The relationship PV = nRT, which

describes the behavior of ideal gases.describes the behavior of ideal gases. Ideal Gas Constant (R):Ideal Gas Constant (R):

A term in the ideal gas law that is used A term in the ideal gas law that is used to make the units work out correctly.to make the units work out correctly.


nRTPV M

mn

M

mRTPV

RT

PMD

V

m


Example:Example: When the temperature of a rigid sphere When the temperature of a rigid sphere

containing 685 L of helium gas is held at containing 685 L of helium gas is held at 621 K, the pressure of the gas is 18,900 621 K, the pressure of the gas is 18,900 kPa. How many moles of helium does kPa. How many moles of helium does the sphere contain?the sphere contain?




V = 685 LV = 685 LT = 621 KT = 621 KP = 18,900 kPaP = 18,900 kPan = ?n = ?





He 2510mol621K

molKkPaL

8.31

685L18900kPaRT

PVn

RT

nRT

RT

PV

nRTPV

Real Versus Ideal GasesReal Versus Ideal Gases

Do real gases behave in ideal ways?Do real gases behave in ideal ways?1.1. As they are compressed, the volume of the As they are compressed, the volume of the

individual real gas particles are significantindividual real gas particles are significant

2.2. Many real gases have intermolecular forcesMany real gases have intermolecular forces

3.3. Most real gases can be liquefied (or Most real gases can be liquefied (or solidified). An ideal gas can’t.solidified). An ideal gas can’t.

* Ideal behavior is most often seen at high * Ideal behavior is most often seen at high temperatures and low pressures.temperatures and low pressures.

Graham’s Law of Diffusion and Graham’s Law of Diffusion and EffusionEffusion

Diffusion:Diffusion: The process by which a gas expands to The process by which a gas expands to

fill the available volume; the random fill the available volume; the random walks of the gas particles take them in all walks of the gas particles take them in all directions. This is why an air freshener directions. This is why an air freshener makes the whole room smell good.makes the whole room smell good.

Effusion:Effusion: The process by which gas particles pass The process by which gas particles pass

through a small opening. This is why air through a small opening. This is why air leaves a tire with a nail hole in it.leaves a tire with a nail hole in it.


Graham’s Law:Graham’s Law: The rate at which gases diffuse or effuse is The rate at which gases diffuse or effuse is

inversely proportional to the square root of inversely proportional to the square root of their molar masses.their molar masses.

1

2

2 gas ofeffusion of rate

1 gas ofeffusion of rate

M

M


Example:Example: A sample of helium effuses through a A sample of helium effuses through a

porous container 6.50 times faster than porous container 6.50 times faster than does an unknown gas. What is the does an unknown gas. What is the molar mass of the unknown?molar mass of the unknown?


Example:Example: A sample of helium effuses through a A sample of helium effuses through a

porous container 6.50 times faster than porous container 6.50 times faster than does an unknown gas. What is the does an unknown gas. What is the molar mass of the unknown?molar mass of the unknown?

RateRate11 = 6.50 = 6.50··RateRate22

MM11 = 4.00 g/mol = 4.00 g/mol

MM22 = ? = ?

mol

g16900.4

1

50.6

rate

rate

rate

rate

rate

rate

rate

rate

2

1

2

2

12

1

2

2

11

1

2

2

1

2

2

2

1

1

2

2

1

MM

MMM

M

M

M

M

M

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The Behavior of Gases Part 2. Ideal Gases Ideal Gas Law: Ideal Gas Law: The relationship PV = nRT, which describes the behavior of ideal gases. The relationship