TBS AP Chem: Dalton's Law of Partial Pressures

Dalton's Law of Partial Pressures

TBS AP Chem

​Dalton's Law of Partial Pressures

​The sum of the individual pressures of all the gases that make up a mixture is equal to the total pressure exerted by the mixture.

​Dalton's Law of Partial Pressures

​The sum of the individual pressures of all the gases that make up a mixture is equal to the total pressure exerted by the mixture.

Multiple Choice

Dalton's Law states that....

• at a constant temperature the volume of a confined ideal gas varies inversely with its pressure.

• the total pressure exerted by the mixture of non-reactive gases is equal to the sum of the partial pressures of individual gases

• gases tend to expand when heated

• the density of an ideal gas at constant pressure varies inversely with the absolute temperature of the gas.

Multiple Choice

Three gases, Ar, N₂, and H₂ are mixed in a 500L container. Ar exerts a pressure of 255 torr, N₂ exerts a pressure of 228 torr and H₂ exerts a pressure of 752 torr. What is the total pressure in the container?

• 483 torr

• 270 torr

• 1235 torr

• 269 torr

​Clues that Dalton's law is involved:

• The gas is collected by water displacement

• The gas is collec​ted over water

• ​The question asks about the "dry gas"

• ​The question gives you the vapor pressure of water

​Collecting gases over water

​For these problems, we often write the equation in this form:

​

​where P_atm = P_tot

​

​You will either be given the vapor pressure of water at that temperature or look it up online

Multiple Choice

A 250 mL sample of oxygen is collected over water at 25° C and 760.0 torr pressure. What is the pressure of the dry gas alone? (Vapor pressure of water at 25° C = 23.8 torr)

• 783.8 torr

• 205.38 torr

• 1250 torr

• 736.2 torr

​Mole Fraction

​The mole fraction of A in a mixture is the moles of A divided by total moles

​

Note: It is always expressed in decimal form with an appropriate number of significant figures.

Fill in the Blanks

A mixture of gases contains 2.00 moles of argon, 4.75 moles of xenon,  and 6.00 moles of neon.  What is the mole fraction of argon in this mixture?

Fill in the Blanks

A mixture contains 11.2 g of helium and 94.0 g of nitrogen gas.  

What is the mole fraction of helium in the mixture?

​Dalton's Law using Mole Fractions

The partial pressure exerted by an individual gas is equal to the mole fraction of the gas times the total pressure.

Fill in the Blanks

A mixture of 3.00 moles of Ar, 4.50 moles of H₂O, 1.75 moles of N₂, and 6.00 moles of CO₂ exerts a total pressure of 800. torr.  What is the partial pressure exerted by the carbon dioxide gas?