1. Gases, cont. (and finished!)
Ideal vs. Real
Ideal Gas Law
Dalton’s Law
Diffusion

2. Chemistry Joke A Chemistry Prison Pun
The silicon put his neon the window ledge.
He climbed out and then krypton along the wall to meet his buddy.
I hope the guard cesium before they argon!

3. Ideal vs. Real Ideal Gases Follow gas laws at all conditions
Conform to the Kinetic Molecular Theory
Insignificant volume
No attraction / repulsion to each other

4. Ideal vs. Real Ideal Gases don’t exist!!! Molecules do take up space
There are attractive forces between them
Otherwise gases would never liquefy
But…at many temperatures and pressures real gases act like ideal gases.
This makes for easier math!

5. Ideal vs. Real We can ignore their volume.
Real Gases behave ideally…
At HIGH TEMPERATURES and LOW PRESSURES…
In these conditions, a gas will stay a gas.
At low pressures, the molecules are far apart.
We can ignore their volume.
At high temperatures, molecules are not together very long.
We can ignore their attractions.

6. Ideal vs. Real Think of Dry Ice
At LOW TEMPERATURES and HIGH PRESSURES…
Gases experience both particle volume and attraction
Can be liquefied or solidified
Think of Dry Ice
We’ll continue to study ideal gases!

7. PV = nRT n represents number of moles R is the universal gas constant
Ideal Gas Law
PV = nRT
n represents number of moles
R is the universal gas constant

8. Units for P and V have to match “R”
Ideal Gas Law
Units for P and V have to match “R”

9. Ideal Gas Law Problems P = ? atm n = 0.412 mol T = 16°C = 289 K
Calculate the pressure in atmospheres of mol of He at 16°C & occupying L.
GIVEN:
P = ? atm
n = mol
T = 16°C = 289 K
V = 3.25 L
R = Latm/molK
WORK:
PV = nRT
P(3.25)=(0.412)(0.0821)(289)
L mol Latm/molK K
P = 3.01 atm

10. Ideal Gas Law Problems V = ? n = T = 25°C = 298 K P = 1.03 atm
Find the volume of 85 g of O2 at 25°C and kPa.
GIVEN:
V = ?
n =
T = 25°C = 298 K
P = 1.03 atm
R =
WORK:
85 g 1 mol = 2.7 mol
32.00 g
2.7 mol
PV = nRT
(1.03)V=(2.7) (0.0821)(298)
atm mol K
V = 64 L

11. Dalton’s Law of Partial Pressures
For a mixture of gases in a container…
P1 represents the “partial pressure” or the contribution by that gas.
Dalton’s Law is particularly useful in calculating the pressure of gases collected over water.

12. Dalton’s Law of Partial Pressures
If the gases in the first three containers are all put into the fourth, we can find the pressure in the 4th container by adding up the pressures in the first 3:

13. Dalton’s Law of Partial Pressures

14. Diffusion
Mixing of molecules
Molecules moving from areas of high concentration to areas of low concentration.
Example: perfume molecules spreading across the room.

15. Diffusion
With diffusion, the mass of the particle is important:
Gases of lower molar mass diffuse faster than gases of higher molar mass.

16. Diffusion Example: compare diffusion rates of helium and nitrogen.
The molar mass of He = 4.0 g
The molar mass of N2 = g
Therefore…
Helium diffuses faster than nitrogen.

17. Chemistry Joke
Q: What did the electron say to the proton to make it unhappy?
A: Something negative!