Proportional Pressures Problems:

Proportional pressure problems involve a container holding a mixture of gases.

In a problem of this type, you are given some combination of amounts and pressures and need to determine the value of a missing factor.

Here is a simple example:
A container holds 3.00 moles of helium, 1.00 mole of hydrogen and 2.00 moles of argon. If the total pressure in the container is 12.00 atm, what is the pressure of the argon? 

We can solve this problem logically.

1. There are 6.00 moles of gas in the container (3.00 + 1.00 + 2.00)
2. The argon makes up 1/3rd of the gas (2.00 of the 6.00 moles)
3. So, the argon must make 1/3rd of the pressure (4.00 atm)

By the same logic, the helium is responsible for 6.00 atm and the hydrogen for 2.00 atm.

Here is another problem using the same idea:
A bottle contains 2.00 moles of helium at a pressure of 4.00 atm. The container also contains xenon. If the total pressure in the bottle is 16.00 atm, how many moles of xenon are in the bottle? 

We know that

1. the helium is responsible for 1/4th of the pressure (4.00atm out of 16.00 atm total) 
2. So, the helium must make up 1/4th of the total number of moles. 
3. If one fourth of the moles = 2.00 moles, there must be a total of 8.00 moles in the container. 
4. The xenon must be the other 6.00 moles.