1. Motion of Gases
Chapter 14.4

2. Calculating the rate of gases
To calculate the rate (speed) you will use the following equation.
Rate =
**R= 8.31 J/mol•K (yes this looks similar to our R value for kPa, but the units are different)
**molar mass (Mm): must be changed to kilograms
Example: 12.0 g/mol = kg/mol
** Temperature (T) : must be in Kelvin
By doing this, the units for our answer end up as m/s.

3. Example What is the speed of Neon atoms when the temperature is 35°C?
rNe =
rNe = 617 m/s

4. What is Graham’s Law?
Relationship between the speed of gas and its molar mass.
Gases with lower molar mass diffuse and effuse faster than gases with higher molar mass.
Review….what is diffusion and effusion?
Example: Hydrogen gas (2.0 g/mol) will diffuse faster than chlorine gas (71.0 g/mol)

5. Comparing Effusion Rates
The rates of effusion of two gases are inversely proportional to the square roots of their molar masses.
Graham’s Law can be written as follows for comparing the effusion rates of two gases.

6. Example
Find which gas will effuse faster. Then use the following equation to solve how many times faster.
Ex.) Helium vs. Nitrogen
So, helium will move 2.7 times faster than nitrogen.