1. The Gas Laws and Stoichiometry
Honors Option for Chemistry
The Gas Laws and Stoichiometry

2. Kinetic Model of Gases The Big Variables are P, V and T
P is created by molecules colliding with the walls of the container

3. T is proportional to the average kinetic energy
V is the space allowed for the gas: remember gases are compressible
T is proportional to the average kinetic energy

4. Boyle’s Law
At a certain, fixed temperature the volume of a gas is proportional to the inverse of the pressure.

5. Charles’ Law
Under constant pressure, as the temperature of a gas increases so does the volume

6. Gay-Lussac’s Law
Under constant volume, the pressure and temperature are proportional

7. Avogadro’s Law
Equal volumes of gases at the same temperature and pressure have the same number of molecules

8. Which Law is It?

9. Combined Gas Law So, why struggle to remember
all those people and laws when
We can summarize them
With the combined gas law.
This law can be used to solve
Problems with ANY of the
Boyle-Charles-Gay-Lussac
Conditions. Just leave out the
Variable that is constant
And only 1 law to remember!

10. Using the Ideal Gas Law What is an ideal gas?
How do gases deviate from the ideal?
No gas is really ideal. However, this law gives good approximation of gas behavior.
It does not work well when the temperature is low (near boiling point) or when the
Pressure is high. Both of those conditions mean that the particles are very close
Together and will experience forces between them. Molecules with high molar masses
Also tent away from the ideal.

11. PV =nRT The ideal gas law!
This law uses the relationships from the combined law and add molar ratio This is often used to find molar mass or density Remember that molar mass is g/mole so g/n And density is mass/volume n/V x g/mole (molar mass)

12. Which Law to Use?
P1V1 = P2V2
T1 T2
PV = nRT
Conditions are changing Example: A piston (volume changes as it moves in and out) A balloon moving up in the atmosphere (the outside P decreases)
Conditions are constant But one variable is unknown Finding the number of moles of a gas at known temperature, volume and pressure (STP for one) Finding any variable for a given chemical reaction

13. Stoichiometry Combined with Gas Laws
Example: For the reaction: 2H2 + O2  2H2O
How many liters of oxygen are needed to react with 2.56 moles of hydrogen at STP?
Find the moles of O2 needed. Use the molar ratio.
2.56 moles mole = moles O2
2 moles
Change the moles to liters
Sub into PV = nRT V = nRT/P
V = (1.28 moles)( L atm/Kmol)(273K)/(1atm)
V = L; with sig figs 28.7 L

14. An unknown gas has a density of 0. 89 g/ml at a pressure of 0
An unknown gas has a density of 0.89 g/ml at a pressure of atm and temperature of 25 C. What is the molar mass?
Use PV = nRT molar mass = g/mol
n/V = P/RT sub in
n/V = (0.900 atm)/( Latm/Kmol)( K)
n/V = mol/L
We want g/mol for molar mass
0.89 g x ml x 1 L = g/mol
ml L mol