Dalton's Law of Partial Pressures
Dalton's law of partial pressures states that the total pressure exerted by a mixture of gases is the sum of partial pressure of each individual gas present. Each gas is assumed to be an ideal gas.
Ptotal = P1 + P2 + P3 + . .
Where P1 and P2 are the partial pressure of gas 1and gas 2 in the mixture. Since each gas behaves independently, the ideal gas law can be used to calculate the pressure of that gas if we know the number of moles of the gas, the total volume of the container, and the temperature of the gas.
Each gas exerts the same pressure they would exert if they were in the container alone. For example, if a mixture of gases at 298 K in a 1.00 L container consists of 1.00 g of H2, 1.50 g of N2, and 2.00 g of Br2. The partial pressure of each gas can be written
The total pressure can be expressed as
The total pressure in this example is 13.8 atm.
Note the following observations about gas mixtures are important to remember.
Each gas occupies the entire volume of the container.The gases will mix homogeneously.
The gases should not react (no chemical reaction should occur between the gases in the mixture).
The type of gas has no bearing on the partial pressure of the gas.
How can two or more gases in a closed container each occupy the entire volume of its container?
Answer: Ideal gases are considered to be point particles, the volume of each gas molecule is considered so small, it is not important. Ideal gases do not attract or repel other gases, therefore the intermolecular forces among the gase molecules is not important. Because gases consist mostly of empty space, the distance between gas molecules is large enough so that each gas molecule does not "see" another gas molecule. This is another reason why there are no interactions with other particles.
The partial pressure of a gas can also be expressed in terms of the mole fraction of each gas.
There is a relationship between the partial pressure of a gas, the total pressure of the system and the mole fraction of the gas.
P1 = X1Pt
