Gas Molecules: Mixtures and movement

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GAS MOLECULES:
MIXTURES AND MOVEMENT
Section 12.5
After reading Section 12.5,
you should know:



Avogradro’s Hypothesis, Dalton’s Law and Graham’s
Law
How to calculate moles, mass, and volume of gases
at STP
How to calculate partial pressures and rates of
effusion
Avogadro’s Hypothesis


Equal volumes of gases at the same temperature
and pressure contain equal numbers of particles
At STP, 1 mole of any gas (regardless of size)
occupies 22.4 L of space
Practice Problem

Determine the volume in Liters occupied by 0.202
mol of a gas at standard temperature and pressure
(STP)?
Known Values:
Have 0.202 moles of gas
1 mol of a gas = 22.4 L
0.202 mol 22.4 L
1 mol
Unknowns:
Vol = ?
= 4.52 L
Dalton’s Law

At constant volume and temperature, the total
pressure exerted by a mixtures of gases is equal to
the sum of the partial pressures of the component
gases.
Practice with Dalton’s Law

Air contains a mixture of oxygen, carbon dioxide, nitrogen
and trace amounts of other gases. What is the partial
pressure of oxygen (O2) at 101.3 kPa of total pressure if
the partial pressures of nitrogen is 79.10 kPa, carbon
dioxide is 0.040 kPa and all other gases is 0.94 kPa?
Known Values:
Ptotal = 101.3 kPa
P N2 = 79.10 kPa
P CO2 = 0.040 kPa
P other gases = 0.94 kPa
Unknowns:
P O2 = ?

Ptotal = P (CO2) + P (O2) + P (N2) + P(other gases)
101.30 kPa = 0.040 kPa + P (O2) + 79.10 kPa + 0.94 kPa
Partial Pressure (O2) = 21.22 kPa
Graham’s Law



Diffusion – is the tendency of molecules to move toward
areas of lower concentration until the concentration is
uniform throughout
Effusion – the process in which a gas escapes through a
tiny hole in it’s container
Graham’s Law – the rate of effusion of a gas is
inversely proportional to the square root of the gas’s
molar mass

Gases with a smaller molar mass will effuse faster than a
gas with a large molar mass
Equation for Graham’s Law

Rate A = √ molar mass B
Rate B
√ molar mass A
Example: Helium has a molar mass of 4.0 grams, nitrogen (N2)
has a molar mass of 28.0 grams. Therefore, helium will
effuse at a faster rate because it is lighter.
Rate He = √ 28.0 g
Rate N2
√ 4.0 g
= 5.3 g = 2.7
2.0 g
Helium will effuse at a rate of
2.7 times faster than nitrogen
After reading Section 12.5,
you should know:



Avogradro’s Hypothesis, Dalton’s Law and Graham’s
Law
How to calculate moles, mass, and volume of gases
at STP
How to calculate partial pressures and rates of
effusion

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