1. Bell Ringer (on Tuesday) A molecule of oxygen gas has an average speed of 12.3 m/s at a given temp and pressure. What is the average speed of hydrogen molecules at the same conditions?

2. Update on Sig Figs Converting oC to K and Dalton’s Law
Keep least number of decimal places (do not count 273 conversion as a factor)
Determine number of sig figs in the following:
Convert -23.0oC to Kelvin.

3. Update on Sig Figs Converting oC to K and Dalton’s Law
Keep least number of decimal places (do not count 273 conversion as a factor)
Determine number of sig figs in the following:
Convert -23.0oC to Kelvin.
-23.0 oC = K

4. Combined Gas Law Quiz

5. Equations So Far…
P1V1 = P2V2 n1T1 n2T2

6. Ideal Gas Law What if there are no changes in P,V,T,n????
Describes the conditions of an ideal gas in terms of pressure, temperature, volume, and the number of moles of gas.

7. UNIVERSAL GAS CONSTANT
Ideal Gas Law PV nT PV T V n
= k
= R
UNIVERSAL GAS CONSTANT
R= Latm/molK
R=8.314 LkPa/molK
You don’t need to memorize these values!

8. UNIVERSAL GAS CONSTANT
Ideal Gas Law
PV=nRT
UNIVERSAL GAS CONSTANT
R= Latm/molK
R=8.314 LkPa/molK
You don’t need to memorize these values!

9. IDEAL GAS LAW PV =nRT USE WHEN CONDITIONS DO NOT CHANGE Pressure P atm
kPa
Volume V L
dm3 OR L
Quantity of gas n
Mol
Gas constant R
(L*atm)/(mol*K)
8.314 (L*kPa)/(mol*K)
Temperature T K

10. Combined OR Ideal Gas Law? When do you use which?
P1V1
n1T1 =
P2V2
n2T2 ?
PV=nRT

11. 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 3.25 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

12. Ideal Gas Law Problems V = ? n = 85 g T = 25°C = 298 K P = 104.5 kPa
Find the volume of 85 g of O2 at 25°C and kPa.
GIVEN:
V = ?
n = 85 g
T = 25°C = 298 K
P = kPa
R = dm3kPa/molK
WORK:
85 g 1 mol = 2.7 mol
32.00 g
= 2.7 mol
PV = nRT
(104.5)V=(2.7) (8.314) (298)
kPa mol LkPa/molK K
V = 64 dm3

13. Real Gases vs. Ideal Gases
What were the assumptions of ideal gases?
Particles very small, gas is mostly empty space.
Particles have no forces of attraction between them.

14. Real Gases
Gases DO have forces of attraction and the molecules DO have volume.
Volume predictions are too low, real volume is higher (due to molecular volume being significant)
Pressure prediction is too high, real pressure is lower (due to molecules having attraction)

15. Making a gas act ideally
Gases can overcome these and act “more ideally” at high temperatures and low pressures.

16. Density of Gases
Big Idea – the molar mass of a gas is directly related to its density.

17. Density of Gases MM = dRT P
Big Idea – the molar mass of a gas is directly related to its density.
MM = dRT P
R = Latm/molK
Density units g/L
Pressure units atm
Temperature units K

18. Example
If the density of a gas is 1.2 g/L at 745 torr and 20. oC, what is its molar mass?

19. Example
If the density of a gas is 1.2 g/L at 745 torr and 20. oC, what is its molar mass?
d = 1.2 g/L
P = 754 torr * (1 atm/760 torr) = .992 atm
T = 20.oC = 293 K
MM = (1.2 g/L)( Latm/molK)(293K)/(.992atm) = 29 g/mol

20. Pre-lab for Ideal Gas Laws Lab
Self-identify the purpose of the lab
Summarize procedure