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.

​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.