1. 11.4 Combined Gas Law Calculations
Ms. Munir

2. Standard Temperature and Pressure
Standard pressure (taken at sea level)is:
760 mm Hg = 760 torr = 1 atm = kPa
Standard temperature is 0°C or 273 K (freezing pt. of water).
Together, this is standard temperature and pressure (STP).
Normal conditions under which we live are known as standard ambient temperature and pressure (SATP), defined as 25°C and 100 kPa.

3. Combined Gas Law Boyle’s Law: Charles’ Law: Gay-Lussac’s Law:
We can combine these:

4. Example 1
On a mild winter’s day, the weather changes and a higher-pressure (103.0 kPa) cold front (−25°C) rushes into town. The original air temperature was −2°C and the pressure was kPa. What will happen to the volume of the 4.2 L balloons?

5. Solution Given: Ti = (−2°C + 273) = 271 K Tf = (−25°C + 273) = 248 K
Pi = kPa
Vi = 4.2 L
Ti = −2°C
Pf = kPa
Tf = −25°C
Vf = ?
Ti = (−2°C + 273) = 271 K
Tf = (−25°C + 273) = 248 K

6. Solution continued…

7. Example 2
In an analysis of 39.8 mg of caffeine using an instrument, 10.1 mL of N2 gas is produced at 23°C and 746 torr. What must the new temperature of nitrogen be, in °C, if the volume is increased to 12.0 mL, and the pressure is increased to 780 torr?

8. Solution Given: Ti = (23°C + 273) = 296 K Pi = 746 torr Vi = 10.1 mL
Pf = 780 torr
Vf = 12.0 mL
Tf = ?
Ti = (23°C + 273) = 296 K

9. Solution continued…

10. Dalton’s Law of Partial Pressures
Dalton’s Law of partial pressures states that the total pressure of a mixture of gases is the sum of the pressures of each of the individual gases.
Ptotal = P1 + P2 + P Pn

11. Example 3
What is the pressure contribution of CO2 to the atmospheric pressure on a very dry day when the barometer read 0.98 atm? Convert your answer into three different units.

12. Solution
What we know:
CO2 contributes 0.03% of the atmospheric pressure. The total atmospheric pressure is 0.98 atm.
The conversion factors of pressure are
1 atm = 760 torr = 760 mm Hg = kPa
0.0003x0.98atm = 2.9 x 10-4 atm
2.9 x 10-4 atm x 101.3kPa/atm = kPa
2.9 x 10-4 atm x 760 Torr/atm = 0.22 mm Hg

13. Work!
P 465 # 1, 3, 4, 5 McGraw Hill
P 562 # 2 – 6 Nelson