1. Temperature and Pressure
General, Organic, and Biological Chemistry
Fourth Edition
Karen Timberlake
Chapter 7
Gases
7.5
Temperature and Pressure
(Gay-Lussac’s Law)
Lectures
© 2013 Pearson Education, Inc.

2. Gay-Lussac’s Law: P and T
Gay-Lussac’s law states that
the pressure exerted by a gas is directly related to the Kelvin temperature.
V and n are constant.
an increase in temperature increases the pressure of a gas. 2

3. Calculation with Gay-Lussac’s Law
A gas has a pressure at 2.0 atm at 18 ˚C. What is the
new pressure when the temperature is increased to
62 ˚C? (V and n constant)
Step 1 Organize the data in a table of initial and final conditions.
Analyze the Problem.
Conditions 1
Conditions 2
Know
Predict
P1 = 2.0 atm
P2 = ?
P increase
T1 = 18 °C + 273
= 291 K
T2 = 62 °C + 273
= 335 K
T increase 3

4. Calculation with Gay-Lussac’s Law
A gas has a pressure at 2.0 atm at 18 ˚C. What is the
new pressure when the temperature is increased to
62 ˚C? (V and n constant)
Step 2 Rearrange the gas law equation to solve for the unknown quantity. 4

5. Calculation with Gay-Lussac’s Law
A gas has a pressure at 2.0 atm at 18 ˚C. What is the
new pressure when the temperature is increased to
62 ˚C? (V and n constant)
Step 3 Substitute values into the gas law equation and calculate. 5

6. Learning Check
Solve for the final pressure of a gas with an initial pressure of 1.20 atm at 75 ˚C when cooled to −22 ˚C.
(V and n constant) 6

7. Solution
Solve for the final pressure of a gas with an initial pressure of 1.20 atm at 75 ˚C when cooled to −22 ˚C.
Step 1 Organize the data in a table of initial and final conditions.
Analyze the Problem.
Conditions 1
Conditions 2
Know
Predict
P1 = 1.20 atm
P2 = ?
P decrease
T1 = 75 °C + 273
= 348 K
T2 = −22 °C + 273
= 251 K
T decrease 7

8. Solution
Solve for the final pressure of a gas with an initial pressure of 1.20 atm at 75 ˚C when cooled to −22 ˚C.
Step 2 Rearrange the gas law equation to solve for the unknown quantity. 8

9. Solution
Solve for the final pressure of a gas with an initial pressure of 1.20 atm at 75 ˚C when cooled to −22 ˚C.
Step 3 Substitute values into the gas law equation and calculate. 9

10. Vapor Pressure
In a closed container, the vapor above a liquid accumulates and creates pressure called vapor pressure. Each liquid exerts its own vapor pressure at a given temperature. As temperature increases, more vapor forms, and vapor pressure increases. 10

11. Vapor Pressure and Boiling Point
A liquid reaches its boiling point when its vapor pressure becomes equal to the external pressure. 11

12. Boiling Point of Water The boiling point of water
depends on the vapor pressure.
is lower at higher altitudes.
is increased by using an autoclave to increase external pressure. 12

13. Learning Check
In which pairs of pressures will water boil? Atmospheric Vapor Pressure Pressure A. 760 mmHg 760 mmHg B. 960 mmHg 760 mmHg C. 520 mmHg 620 mmHg 13

14. Solution
In which pairs of pressures will water boil? Atmospheric Vapor Boiling Pressure Pressure Occurs? A. 760 mmHg 760 mmHg yes B. 960 mmHg 760 mmHg no C. 520 mmHg 620 mmHg yes 14