Dalton's Law of Partial Pressures

QuickNotes

           Dalton's Law of Partial Pressures: P_total = P₁+P₂+P₃+...

• Dalton's law summarized is: 
• For a mixture of gases in a container, the total pressure exerted is the sum of the pressure that each gas would exert if it were alone.
• P refers to pressure measured in atmospheres (atm)

• The partial pressure of a gas can also be expressed in terms of the mole fraction of each gas.

Important Concepts: 

1. Each gas occupies the entire volume of the container.
2. The gases will mix homogeneously. 
3. The gases should not react (no chemical reaction should occur between the gases in the mixture). 
4. The type of gas has no bearing on the partial pressure of the gas.

Examples: 

Example 1: The partial pressure of oxygen was observed to be 156 torr in air with a total atmosphere pressure of 743 torr. Calculate the mole fraction of oxygen present. 

• First, list out the variables you know.
• 156 torr of oxygen.
• Total pressure is 743 torr.
• Then, solve for the mole fraction of oxygen present using the Dalton's Law of Partial Pressures expressed in mole fraction.

XO2 = PO2 /Ptotal = 156 torr/743 torr
= 0.210
*Note that the mole fraction has no units

Example 2: A sample of gas A evaporates over water in a closed system. What is the pressure of gas A if the total pressure is 780 torr and water vapor pressure is 1 atm?

• Convert pressure to same units so:
• 780 torr =1.03 atm
• Subtract water vapor pressure from total pressure to get partial pressure of gas A: 
• P_A= 1.03 atm - 1 atm = 0.03 atm

To make sure you completely understand Dalton's Law, check out this review video!

Do you think you are ready? Get out a calculator and take a mini-quiz. If you get a 100%, move onto the next concept! (Make sure you have an additional tab open to play at kahoot.it. You will need to keep 2 tabs open to play and view the questions.)

Dalton's Law Quiz