1. Density, Dalton Avogadro & Graham

2. Density of Gas
The density of a gas varies widely depending on temperature and pressure
Remains the same
D = m/v

3. Hints to Density problems
Write down everything known and unknown in the problem
Before you find D you must find V2
If D is given, immediately find mass to volume ratio

4. Dalton’s Law PTotal = P1 + P2 + P3 . . . and
In a rigid container (same volume) where a mixture of different gases is present, the total pressure of the mixture is equal to the sum of the pressures for each of the gases
PTotal = P1 + P2 + P
and

5. Number of moles is proportional to pressure…
Number of moles is proportional to pressure…. if they are in the same system.
2 mol of a gas exerts twice as much pressure as 1 mol

6. Practice
In a fixed container, 3 mol of CO2 and 2 mol of O2 exist at a temp of 298K and a pressure of 45kPa. What is the partial pressure exerted by the oxygen?

7. Answer 3 mol CO2 + 2mol O2 = 5 mol of gas
5 mol gas = total pressure 45kPa
If 5x = 45 then X = 9
Then 2 mol O2 = 18kPa
And 3 mol CO2 = 27kPa

8. Avogadro’s Law
Equal volumes of gases under the same conditions of temperature and pressure contain the same number of molecules.

9. CO2 and CH4 are in separate containers. CO2 occupies a volume of 3
CO2 and CH4 are in separate containers. CO2 occupies a volume of 3.5L at 298K and 105kPa. CH4 (methane) occupies a volume of 3.2L at 273K and 120kPa. Which container contains more gas molecules?

10. CO2
V1 = 3.5L
T1 = 298K
P1 = 105kPa
T2 = 273K
P2 = 120kPa
Set the second temperature and pressure the same as the CH4.
Use combined gas law to calculate V2 of CO2
The greater volume has more molecules.
V2 of CO2 = 2.81L so CH4 has more molecules
CH4
V= 3.2L
T = 273K
P = 120kPa

11. Another way to solve the problem . . .
Use the ideal gas law to determine the number of moles for each. Greater number of moles = greater number of molecules.
PV = nRT for carbon dioxide
(105kPa)(3.5L) = n(0.0821)(298)
n= mol
PV = nRT for methane
(120kPa)(3.2L) = n(0.0821)(273)
n= mol

12. Graham’s Law
Molecules of higher molecular mass undergo diffusion & effusion more slowly as compared to molecules of lower molecular mass.

13. Diffusion
Movement of particles from an area of high concentration to low concentration until the concentration is uniform
Effusion
Escape of a gas through pores of its container

14. Graham’s Equation
Suppose you have two balloons, one filled with helium and one balloon filled with CO2.
Which will deflate faster? How much faster will it deflate?

15. The End