Gas Molecules: Mixtures and movement
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: Unknowns:
Have 0.202 moles of gas Vol = ?
1 mol of a gas = 22.4 L
0.202 mol 22.4 L = 4.52 L
1 mol
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 (O
2
) 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:
P total
P
N2
= 101.3 kPa
= 79.10 kPa
P
CO2
= 0.040 kPa
P other gases
= 0.94 kPa
Unknowns:
P
O2
= ?
P total
= 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
Rate
B
= √ molar mass
B
√ molar mass
A
Example: Helium has a molar mass of 4.0 grams, nitrogen (N
2
) has a molar mass of 28.0 grams. Therefore, helium will effuse at a faster rate because it is lighter.
Rate
He
Rate
N2
= √ 28.0 g
√ 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
