General Chemistry 1

1. Unit 5 - Gases
Objective :
1. What are the fundamental properties of
gases?
2.What laws best describe their behavior?
3.What theory explains these properties
and laws?

2. The amount of air we breath
Everyday we move about 8500 L of air in to and out
of your lungs.
The total mass of this air is 11kg (25lb)
How we can measure this air?
The answer is pressure

3. pressure
•It’s the force exerted per unit area by gas
molecules as they strike the surface around
them
•The higher the concentration of the gas
molecules, the greater will be the pressure.

4. Pressure
•“Molecular Push”
•Formula: Pressure (P = force/area)
Or P=F/A
•SI Unit: Newtons per square meter (N/m2)
•Pascal (N/m2), kilopascal (kPa)
•Pressure is measured using a barometer

5. Pressure
•Other units: mm Hg, atm, torr, bar, psi(pounds
per square inch)
•1 atm = 101.325 kPa
•1 atm = 760 mm Hg
•1 mm Hg = 1 torr
•1 atm = 1.013 25 bar
•1 atm= 14.696 psi

6. Pressure Conversions
•1) Convert 789 mm Hg to atm
•2) A Canadian weather report gives the
atmospheric pressure as 100.2 kPa.
What is the pressure in Torr?
•3) What is the pressure in a rectangular
container if the particles exert a force of
120 N? The container is 0.4 m long and
0.23 m wide.

7. Pressure Devices
• A. Barometer
• Measures atmosphere pressure in
mmHg
•B. Manometer
•Types: Open & Closed
•Mercury is used because of its density
( = 13.6 g/cm3)

8. manometer

9. Boyle’s Law
•For a given amount of gas at constant
temperature, the volume of the gas varies
inversely with its pressure
•Mathematically P1V1=P2V2

10. Pressure – Volume Relationship

11. Video – Boyle’s law
• https://www.khanacademy.org/test-prep/mcat/physical-
processes/gas-phase/v/boyles-law

12. Boyle's Law
0
100
200
300
400
500
600
0 100 200 300 400 500
Pressure (kPa)
Volume
(L)

13. Example
•1) A sample of helium occupies 535 mL at
988 Torr and 25˚C. If the sample is
transferred to a 1.05 L flask at 25˚C, what
will be the gas pressure in the flask?
•Answer= 503 Torr

14. Example
2) A sample of air occupies 73.3 mL at 98.7 kPa
and 0˚C. What volume will the air occupy at
4.02 atm and 0˚C?
• Answer: 17.8 mL

15. Warm -Up
•Why the second floor of the house is usually
warmer than the first floor?
• Answer is Charles's Law

16. Charles’s Law
•Jacques Charles found that temperature
and volume were directly related which
means that if the volume goes up so does
the temperature.
•Volume is proportional to temperature
• You must use Kelvin
• K= 273 + ˚C
• If you double the temperature you
double the volume
2
2
1
1
T
V
T
V
=

17. Charles's Law
0
500
1000
1500
0 200 400 600
Temperature (C)
Volume
(mL)

18. Charles’s Law Problem
• A sample of hydrogen occupies 692 L at 602°C.
If the pressure is held constant, what volume will
the gas occupy after cooling to 23°C?

19. Example
•A balloon at indoors, where the
temperature is 27˚C, has a volume of 2.00 L.
What will its volume be outdoors, where
the temperature is -23˚C?
• Answer: 1.67 L

20. Review Problem
•If the gas present in 4.65 L at STP is changed
to a temperature of 15˚C and a pressure of
756 Torr, what will be the new volume?

21. Combined Gas Law
•Combines all three variables
(pressure, volume,
temperature)
•Temperature must still be in
Kelvin

22. Example
•A cylinder of gas is kept at a constant
volume, as the temperature increases from
24.1 ˚C to 326.4 ˚C. If the initial pressure is
1.01 atm, what is the final pressure in
mmHg?

23. Example of Combined Gas Law
•A gas occupies of a volume of 459 mL at
23ºC and 234 torr. What is the gas’s new
volume if the temperature increases to 45ºC
and the pressure doubles?

24. Example # 2
•Aerosol containers often carry the warning
that they should not be heated. Suppose
such a container were filled with a gas at 2.5
atm and 22 ˚C, and suppose that the
container may rupture if the pressure
exceeds 8.0 atm. At what temperature is
that rupture likely to occur?

25. Example # 3
•A 350 mL sample of helium gas is collected
at 22.0 oC and 99.3 kPa. What volume
would this gas occupy at STP?

26. STP
•Standard Temperature and Pressure
•Standard Temperature
•273 K or 0˚C
•Standard Pressure
•1 atm or 101.3 kPa

27. Example
•A gas occupies of a volume of 634 mL at
30ºC and 335 kPa. What is the gas’s new
volume if the temperature increases to 45ºC
and the pressure goes to 800 kPa?

28. Avogadro's Law
•What happens when the amount of gas
changes?
•As amount of gas increases, volume
increases
• V α n (at constant T and P)

30. Consider the inflation of a balloon

31. Problem
•A chemical reaction occurring in a cylinder
equipped with a movable piston produces
0.621 mole of a gaseous product. If the cylinder
contained 0.120 mol of gas before the reaction
and had an initial volume of 2.18L, what was its
volume after the reaction? (Assume constant
pressure and temperature and that the initial
amount of gas completely reacts)

32. Warm up
•A 56.0 mL sample of neon gas is at the same
pressure as the prevailing barometric
pressure. If the pressure of the gas in
increased to 100.0 Torr above the
barometric pressure while the temperature
is held constant, the volume is observed to
change to 49.4 mL. What is the prevailing
barometric pressure?

33. Example
•If all of the Helium is used to fill 1.5 L red
balloons at 0.99 atm and 32 ˚C, how many
balloons will you end up with?

34. Ideal Gas Equation
•Combining Boyle’s law, Charles's law and
Avogadro’s law

35. Ideal Gas Law

36. •P = pressure (atm)
•V= Volume (L)
•n= number of moles
•R= 0.0821 L*atm/ K* mol
•T= temperature (K)

37. Example
•What is the pressure in atmospheres
exerted by a 0.500 mole sample of nitrogen
gas in a 10.0 L container at 298 K?

38. •A 8.5 L tank contains Helium gas at 16.9 atm
and 25˚ C. How many moles of He are
available for making balloons?
•If all of the Helium is used to fill 1.5 L red
balloons at 0.99 atm and 32 ˚C, how many
balloons will you end up with?

39. Example
•An unknown gas is collected in an 850.9 mL
vessel at 1.02 atm and 23 ˚ C. The evacuated
vessel has a mass of 138.45 g, and the vessel
and gas have a combined mass of 140.12 g.
Find the molar mass of the gas
• Answer: 0.0357 mol

40. Example
•If 12.0 g O2 are required to inflate a 500 cm3
tire at 30°C, what pressure is present inside
the tire? Assume the gas behaves ideally.

41. Additional Problems
•How many moles of N2 gas are in 0.38 L at
0ºC and 380 mm Hg?
•What volume would 25.0 grams Argon
occupy at 90°C and 735 torr?

42. Additional Problems
•An electronic vacuum tube was
sealed off during manufacture at a
pressure of 1.8 x 10-5 torr at 27°C.
Its volume is 100 cm3. Compute
the number of gas molecules
remaining in the tube.