1. 12.1 Combined Gas Law & Avogadro’s Hypothesis and Molar Volume
UNIT 5: GASES
12.1 Combined Gas Law & Avogadro’s Hypothesis and Molar Volume

2. CONTEXT So far we have manipulated 3 variables: T P V
What are some limitations of the three laws we have studied?
What has to be kept constant?

3. COMBINED GAS LAWS
In Boyle’s, Charles’s, and Gay-Lussac’s Laws, number of moles and another variable must always be kept constant
The problem: in real life situations, often all three variable (T, P, V) are changing
The laws can be combined to allow changes in all 3 properties – T, P, V
P1V1 = P2V2
T T2

4. PRACTICE
A balloon contains 275 mL of helium gas at 25.0°C and pressure of 350kPa. What is the volume at 10.0°C and pressure of 101kPa?
P1=
V1=
T1=
T2=
P2=
V2=
350 kPa
275 mL
25.0°C
= K
= K
101 kPa ?
P1V1 = P2V2
T1 T2
V2 = P1V1T2
T1P2
V2 = 350 x 275 x
x 101 kPa
V2 = 905 mL

5. Even with the combined gas law, the number of moles remains constant
What happens when we change the number of moles of a gas?

6. AVOGADRO’S LAW
The volume of a gas is directly proportional to the amount of gas (moles), when temperature and pressure remain constant. V  n V = kn V = k n V1 = V2 n1 n2

7. MOLAR VOLUME
Molar Volume (v) – The volume that one mol of gas occupies at a specific temperature and pressure
Units: L/mol
v = V/n
At STP – Standard Temperature and Pressure (O˚C, kPa) molar volume is 22.4 L/mol
At SATP – Standard Ambient Temperature & Pressure (25˚C, 100 kPa) molar volume is 24.8 L/mol

8. Homework
Worksheet questions
P. 477 # 1-3
P. 482 # 5-8