1. Plus, the density formula and Avogadro’s Law
The Ideal Gas Law
Plus, the density formula and Avogadro’s Law

2. Review Gases are made of particles that move rapidly and randomly.
Temperature is a measure of how rapidly the molecules in a gas are moving on average.
Collisions between gas molecules surface of an object (or the walls of a container) give rise to gas pressure.
Standard temperature and pressure
273 K (0ºC)
kPa (1 atm)

3. Ideal Gases
An ideal gas is one that perfectly obeys the predictions made by the KMT:
Its molecules have zero diameter.
Its molecules have zero intermolecular forces.
Collisions are always elastic.
Gases aren’t perfectly ideal.
Some gases approach “idealness” under certain conditions:
high temperature
low pressure

4. Ideal vs. Real Gases
The boiling point of nitrogen (N2) is Kelvins ( ºC).
At room temperature (298 K) and standard pressure (1 atm), nitrogen behaves very much like an ideal gas.
Room temperature is far above nitrogen’s boilling point.
At 78 K and 30 atm, nitrogen’s behavior isn’t so ideal.
Its molecules are just barely moving fast enough to remain in the gas state.
Intermolecular forces affect the behavior of N2 at such a low temperature.
High pressures squeeze the molecules close together, increasing the effects of their intermolecular forces.

5. Ideal vs. Real Gases
Ideal Gas

6. Ideal vs. Real Gases
In a gas that approximates ideal behavior, molecules are fast-moving and far apart.
At very low temperatures, gas molecules become “sluggish” and attractive forces alter their behavior.
At very high pressures gas molecules are squeezed close together and molecular interactions become far more common (and important).

7. The Ideal Gas Law PV = nRT
P = pressure (atm)
V = volume (L)
n = moles
R = Universal Gas Constant
R = L*atm/mol*K
T = temperature (K)
Applies to gases that exhibit ideal behavior.
For non-ideal gases (gases at very low temperature or extremely high pressures) PV ≠ nRT.

8. The Ideal Gas Law In order to use R = 0.0821 L*atm/mol*K,
Pressure must be expressed in atm.
Volume must be expressed in L.
Temperature must be expressed in K.
You should get used to converting between different units of pressure, volume, and temperature.
There are other values of R for use with other units, but instead of learning many different values for R, you should learn how to convert.

9. The Ideal Gas Law
What is the volume of 3.00 moles of helium at a temperature of 400. K and a pressure of 1.50 atm?
PV = nRT
(1.50 atm) V = (3.00 mol)( L*atm/mol*K) (400. K)
(1.50 atm) V = 98.5 L*atm
V = 65.7 L

10. The Ideal Gas Law
What is the pressure exerted by moles of nitrogen gas in a 45.0 L container if the temperature is 350.ºC?
First we must convert ºC to K?
350.ºC = 623 K
PV = nRT
P(45.0 L) = (0.400 mol)( L*atm/mol*K)(623 K)
P(45.0 L) = 20.5 L*atm
P = atm

11. The Ideal Gas Law
At what temperature (in ºC) will 2.00 grams of argon gas exert a pressure of 12.5 atm in a 160.-mL container?
First we must convert from grams of Ar to moles:
2.00 g Ar x = mol Ar
Next, convert from mL to L:
160. mL x = L
g Ar
1 mol Ar
1000 mL
1 L

12. The Ideal Gas Law
At what temperature (in ºC) will mol of argon gas exert a pressure of 12.5 atm in a L container?
PV = nRT
(12.5 atm)(0.160 L) = ( mol)( L*atm/mol*K) T
2.00 atm*L = ( L*atm/K) T
T = 486 K
T = 213º C

13. Gas Density Density = mass / volume The volume of an ideal gas is:
V = nRT / P
Putting the two equations together:
D = mP / nRT
The molar mass of a gas is the number of grams (m) per mole (n), so:
Molar mass (M) = m/n
For an ideal gas, D = PM / RT

14. Gas Density
What is the density of chlorine gas, Cl2, at atm and 300. K?
The molar mass of Cl2 is:
2 x Cl = 2 x g/mol = g/mol
D = PM / RT
D = (0.950 atm)( g/mol) / ( L*atm/mol*K)(300. K)
D = 2.73 g/L
1 Liter of Cl2 under these conditions would weigh 2.73 grams.

15. Gas Density What is the density of CO2 gas at STP? D = PM / RT
The molar mass of CO2 is:
1 x C = 1 x g/mol = g/mol
2 x O = 2 x g/mol = g/mol
Total = g/mol
Standard temperature = 273 K
Standard pressure = 1.00 atm
D = PM / RT
D = (1.00 atm)( g/mol) / ( L*atm/mol*K)(273 K)
D = 1.96 g/L

16. Avogadro’s Law
In addition to having a very large number named after him, Amedeo Avogadro made a very important deduction about gases.
Equal volumes of ideal gases at equal temperatures and pressures contain equal numbers of molecules.
Doesn’t matter what the identity of the gas is.
In other words, if you have two different ideal gases under identical conditions, the molar volume of the gases is the same.
At STP, 1 mole of any gas = 22.4 L

17. Avogadro’s Law
What is the volume of 2.00 moles of O2 at STP? (Assume ideal behavior.)
1 mol of any STP = 22.4 L
2.00 mol O2 x = 44.8 L O2
1 mol O2
22.4 L O2

18. Avogadro’s Law

19. A Reminder
Gas law problems don’t always have to come with easy-to-use units.
They often require unit conversions before they can be solved.
You should always ask yourself “What does this problem want me to find, and what am I given?”
Make a plan to solve the problem based on the available information.
Not every problem will be the same.
If you approach all gas law problems expecting them to be identical, you will be frustrated.
Don’t get lazy or sloppy in your work!