1. Gases Chapter 5

2. Things to know about gases
Have no specific volume-volume of the gas is the volume of the container
Temperature must be in Kelvin
Barometers measure atmospheric pressure
760 mm Hg
1 atm
101.3 kPa
STP

3. Charles’ Law
Volume is directly proportional to absolute temperature

4. Boyle’s Law Volume is inversely proportional to pressure P1V1 = P2V2

5. Gay-Lussac’s Law
Pressure is directly proportional to absolute temperature

6. Ideal Gas Law P = pressure (atm) V = volume (L) n = moles R = 0.0821
T =temperature (K)
PV = nRT

7. What is the pressure exerted by 15. 0 mol of O2 in a 50
What is the pressure exerted by 15.0 mol of O2 in a 50.0 L tank at 50 C?

8. GMM = molar mass
d = density = mass/volume (g/L)
R =
T = temperature (K)

9. Determine the density of O2 at 27 C and 735 mm Hg

11. Density of a gas Compressing a gas increases its density
Increasing temperature decreases density--hot air rises
Hydrogen is the least dense gas of all because it has the lowest GMM--greatest lifting power

12. A flask weighs 52. 693 g empty and 53
A flask weighs g empty and g when filled with acetone vapor at 100 C and 752 mm Hg. Taking the volume of the flask to be mL calculate the molar mass of acetone.

16. Gases in reactions
First law of Gases. . .Gas Laws only work for Gases!

17. Determine the mass of zinc metal required to react with HCl to form 16
Determine the mass of zinc metal required to react with HCl to form 16.0 L H2 (g) at 20 C and 735 mm Hg

19. Dalton’s Law of Partial Pressures
The total pressure of a gas mixture is the sum of partial pressures of the components of the mixture.
Ptotal = P1 + P
Used when gases are collected over water
Pgas =Ptotal - Pwater vapor
Pwater vapor is the vapor pressure of water at the specified temperature.

20. A student prepares a sample of hydrogen gas by electrolyzing water at 25 C. She collects 152 ml of hydrogen over water at a pressure of 758 mm Hg, calculate the moles of hydrogen collected.
Look up the vapor pressure of water at 25 C from appendix 1.
Pwater = mm Hg.
758 mm Hg – mmHg = 734 mm Hg

21. Now solve for moles of hydrogen with the ideal gas law.

22. Partial pressure and mole fractions
Mole fraction = X
Xa = na/ntotal
Partial pressure of a gas in a mixture is equal to its mole fraction multiplied by the total pressure
Pa = XaPtotal

23. Find the partial pressure of oxygen in air when the barometric pressure is 734 mm Hg and the mole fraction of oxygen is

24. Kinetic theory of Gases
Gases are mostly empty space
Gas molecules are in constant random motion
They collide with each other and the sides of the container
Collisions are elastic
Gas pressure is caused by the collisions with the walls of the container
Increase P increases the number of collisions

25. Expression for pressure
The ratio N/V is the concentration of gas mc in the container. The more mc, the more collisions, the more P.
mu2 is the measure of the energy of the collisions.

26. Average kinetic energy of translational motion Et
At a given T molecules of different gases must all have the same Et
Et of a molecule is directly proportional to the T in Kelvin
T is the only variable!
R =
NA = X 1023

27. Average speed, u

28. Constant GMM aka the same gas

29. At constant temperature

30. Graham’s Law
Effusion
The flow of gas molecules at low pressures through tiny pores or pinholes.
At a given temperature the rate of effusion of a gas is inversely proportional to the square root of its molar mass

31. What does this mean? A lighter gas will have a faster rate.
Can be used to separate isotopes

32. Real gases
The closer a gas is to the liquid state (high P low T) the more it will deviate from the ideal gas law because
Of the attractive forces between gas molecules
Gives a smaller molar volume (<22.4 L/mol) than predicted
The larger the attractive forces the bigger the deviation
The finite volume of gas particles
Gives a larger molar volume (>22.4 L/mol) predicted because gases can only get so close