1. The Gas Laws

2. Pressure Force per unit area. Gas molecules fill container.
Molecules move around and hit sides.
Collisions are the force.
Container has the area.
Measured with a barometer.

3. Vacuum
Barometer
The pressure of the atmosphere at sea level will hold a column of mercury 760 mm Hg.
1 atm = 760 mm Hg
760 mm Hg
1 atm Pressure

4. Units of pressure 1 atmosphere = 760 mm Hg 1 mm Hg = 1 torr
1 atm = 101,235 Pascals
= kPa
Can make conversion factors from these.

5. What is 724 mm Hg in torr?
724 torr
96.5 torr
0.953 torr
73,359 torr

6. What is 724 mm Hg in kPa?
724 kPa
96.5 kPa
0.953 kPa
73,359 kPa

7. What is 724 mm Hg in atm?
724 atm
96.5 atm
0.953 atm
73,359 atm

8. The Gas Laws Boyle’s Law
Pressure and volume are inversely related at constant temperature.
PV= k
As one goes up, the other goes down.
P1V1 = P2 V2
Graphically

9. V
P (at constant T)

10. Slope = k V
1/P (at constant T)

11. Examples
20.5 L of nitrogen at 25ºC and 742 torr are compressed to 9.8 atm at constant T. What is the new volume?
2.04 L
.264 L
.467 L

12. 30.6 mL of carbon dioxide at 740 torr is expanded at constant temperature to 750 mL. What is the final pressure in kPa?
30.2 kPa
.0397 kPa
4.025 kPa
kPa

13. Charles' Law
Volume of a gas varies directly with the absolute temperature at constant pressure.
V = kT (if T is in Kelvin)
V1 = V T1 = T2
Graphically

14. He
CH4
H2O
V (L) H2 ºC
T (ºC)

15. Examples
What would the final volume be if 247 mL of gas at 22ºC is heated to 98ºC , if the pressure is held constant?
1100 mL
196 mL
311 mL

16. At what temperature would 40.5 L of gas at 23.4ºC have a volume of 81.0 L at constant pressure?
592.8 K C
319.8 K

17. Avogadro's Law
At constant temperature and pressure, the volume of gas is directly related to the number of moles.
V = k n (n is the number of moles)
V1 = V n1 = n2

18. Gay- Lussac Law
At constant volume, pressure and absolute temperature are directly related.
P = k T
P1 = P T1 = T2

19. Combined Gas Law P1 V1 = P2 V2 . T1 T2
If the moles of gas remains constant,
use this formula and cancel out the
other things that don’t change.
P1 V1 = P2 V T T2

20. Ideal Gas Law
PV = nRT
V = L at 1 atm, 0ºC, n = 1 mole, what is R?
R is the ideal gas constant.
R = L atm/ mol K
Tells you about a gas is NOW.
The other laws tell you about a gas when it changes.

21. Ideal Gas Law A hypothetical substance - the ideal gas
Think of it as a limit.
Gases only approach ideal behavior at low pressure (< 1 atm) and high temperature.
Use the laws anyway, unless told to do otherwise.
They give good estimates.

22. Example
A 47.3 L container containing 1.62 mol of He is heated until the pressure reaches 1.85 atm. What is the temperature?
316 K
658 K
27.1 K

23. Gas Density and Molar Mass
D = mass/Volume
Let MMstand for molar mass
MM = mass/n
n= PV/RT
PV = mass RT MM

24. Example What is the density of ammonia at 23ºC and 735 torr? 0.676 g/L

25. Gases and Stoichiometry
Reactions happen in moles
At Standard Temperature and Pressure (STP, 0ºC and 1 atm) 1 mole of gas occupies L.
If not at STP, use the ideal gas law to calculate moles of reactant or volume of product.