1. Chemistry 141 Friday, October 13, 2017 Lecture 17 Ideal Gases
Chemistry 11 - Lecture 11
9/30/2009
Chemistry 141
Friday, October 13, 2017
Lecture 17
Ideal Gases

2. Concept Question
What will happen to the volume of an inflated balloon when it is placed in liquid nitrogen (77 K)?
goes up
goes down
stays constant

3. Charles’s Law
The volume of a gas is proportional to its temperature
decrease in T
decrease in V
V = bT + C
Copyright © 2002 by Houghton Mifflin Company. All rights reserved.

4. Absolute Temperature
V = bT + C
V = bT
We can define a new scale of temperature such that the zero of temperature is the point where the gases have zero volume. This temperature is called absolute zero.
Absolute zero = °C T(K) = T(°C)
Copyright © 2002 by Houghton Mifflin Company. All rights reserved.

5. Avogadro’s Hypothesis
Gay-Lussac noted that the volumes of gases at same T and P that react together are in the ratio of small whole numbers
Avogadro concluded that equal volumes of gases contain equal numbers of particles (at same T and P)
The volume a gas occupies is proportional to the number of particles
n = # of particles in 1 volume
a = a constant
V = an
2V = a(2n)

6. Combining the gas laws Boyle’s Law: Charles’s Law: Avogadro’s Law:
combine constants into R
The ideal gas law

7. Using the Ideal Gas Law
An ideal gas is in a cylinder with a volume of 5.0×102 ml at a temperature of 30. °C and a pressure of 710 torr. The gas is compressed to a volume of 25 ml, and the temperature is raised to 820 °C. What is the new pressure?

8. The gas constant
The gas constant tells us how much P or V will change if n or T changes
Standard temperature and pressure (STP)
P = 1 atm
T = 0 °C (= K)
What volume is occupied by 1 mole of an ideal gas at STP?

9. Gas stoichiometry example
Metallic molybdenum can be produced from the mineral molybdenite, MoS2. The mineral is first oxidized in air to MoO3 and SO2. MoO3 is then reduced to metallic Mo using hydrogen gas. The balanced equations are:
MoS2 (s) + O2 (g)  MoO3 (s) + 2SO2 (g)
MoO3 (s) + 3H2 (g)  Mo (s) + 3H2O (l)
Calculate the volume of oxygen gas at 17 °C and 1.00 atm that is needed to produce 1.00×103 kg of pure Mo from MoS2.

10. Concept question
You have two containers of gas at the same temperature and pressure. One contains ammonia gas, and the other air. Which is lighter, and why?

11. Gas density
The density of a gas is related to its pressure, temperature, and volume
(density = d or ρ)
m = mass
Mx = molar mass of X
the density of a gas depends on its molar mass

12. Questions
The amount of CO2 in the atmosphere is changing with time. How much CO2 is there and how big is the change?
Do increases in CO2 lead to changes in air properties, such as density?

13. Gas mixtures Dalton’s Law of Partial Pressures
What happens when two different gases are mixed together?
Consider equal volumes at constant T:
Remember: P, V, and T are independent of the identity of the gas – they depend only on the total n
how do the pressures of these samples relate to each other? + =
ni nj = ntotal
pressures add!
Dalton’s Law of Partial Pressures

14. Mole fraction
We define a new measure of concentration for gas mixtures, the mole fraction:
since