1. Atmospheric Pressure
Pressure is equal to a force per area. The gases in the air exert a pressure called atmospheric pressure.
Atmospheric pressure is a result of all of the collisions that result from air particles bumping into things as they move around.
Atmospheric pressure is measured with a barometer.

2. Units for Pressure mmHg (millimeters mercury) or torr
1.000 atm = mmHg = torr
1.000 atm = 101,325 pascal (Pa)
1.000 atm = pounds per square inch (psi) SI
UNIT!
USED IN ENGINEERING!

3. Gases can be described using the following four variables:
Properties of Gases
Gases can be described using the following four variables:
V = volume of the gas (liters, L)
P = pressure (atmospheres, atm)
T = temperature (Kelvin, K)
n = amount (moles, mol)
UNITS

4. Reading Quiz Gas Laws What is pressure?
Convert 687 torrs into atmospheres.
If the temperature (T) and number of moles (n) are constant, what happens to volume (V) as pressure (P) increases?
What if P decreases?

5. ANSWERS P & V are ? P V related! P V F A indirectly
Pressure (P) is force per unit area
687 torr atm = atm
760 torr
If T is constant…
as P increases, V decreases!
as P decreases, V increases!
P & V are ?
related!
P V
indirectly
P V

6. P may change and V may change, but their product stays the same!
Boyle's Law
P1 V1 = P2 V2
P may change and V may change, but their product stays the same!

7. Boyle's Law P1 V1
T & n
constant

8. P2 V2 T & n Boyle's Law If pressure increases, volume decreases!
constant
If pressure increases, volume decreases!
(when temperature’s constant)

9. V2 P2 T & n Boyle's Law If pressure decreases, volume increases!
constant
If pressure decreases, volume increases!
(when temperature’s constant)

10. Inverse and Direct Proportions… which is which?B

11. Inversely Proportional Ex: Boyle’s Law (V as a function of P)
P (torr)
V (mL)
100
4560
200
2280
300
1520
400
1140
500
912
600
760
700
651
800
570
900
507
1000
456
1100
414
1200
380
1300
351
1400
326
1500
304

12. Boyle’s Law at Different Temperatures (P as a function of V)

13. INVERSE RELATIONSHIPS
As one variable goes up, the other goes down!
Produces a
curved graph…
Multiplying the two variables equals a constant. P V T
constant
P1V1 = P2V2

14. Directly Proportional Ex: Charles’ Law (V as a function of T)
T (K)
V (mL) 50
100
200
150
300
400
250
500
600
350
700
800
450
900
1000
550
1100
1200
650
1300
1400

15. DIRECT RELATIONSHIPS T As one variable goes up, so does the other! V
Produces a straight
line graph…
Dividing the one variable
by the other
equals a constant.
V1 V2
T T2 =

16. Want Practice?:
V (L)
n (mol) 5 10 15 20
1.0 25 30 35 40 45 50 55 60 65 70 75
Complete the following data table for Avogadro’s Law (Volume is constant)
Graph your data (properly).
Are V & n inversely or directly related? Why?

17. Directly Proportional (V as a function of n)
Avogadro’s Law:
Directly Proportional (V as a function of n)
V (L)
n (mol)
5.0
0.250
10.
0.500 15
0.750
20.
1.00 25
1.25
30.
1.50 35
1.75
40.
2.00 45
2.25
50.
2.50 55
2.75
60.
3.00 65
3.25
70.
3.50 75
3.75
V V2
n n2 =