Chapter 6: Unit 7. Dalton’s Law

Dalton’s Law

In a mixture of gases, each molecule acts independently of all the others, provided that the gases behave as ideal gases and do not interact with each other in any way.

Dalton’s law of partial pressures states that the total pressure exerted by a mixture of gases is the sum of the partial pressures of the individual gases.

Mathematically we can write,

Example” A sample of exhaled air from the lungs contains four gases with the following partial pressures. N2( 517 mm of Hg), O2(15. mm of Hg) and CO2(50. mm of Hg). What is the total pressure of the sample?

Ans: 517+15+50= 582 mm of Hg

Partial pressure of a gas can be calculated using mol fraction of a gas in a gas mixture and total pressure.

formula of MOLE FRACTION= n_A/n_total

Total pressure = 1 atm

For example: in the above diagram, total number of gas molecules =14. Number of A (blue) gas molecules= 7, number of B ( green) gas molecule= 7.

Mole fraction gas A= 7/14= 0.5

Mole fraction of B= 7/14= 0.5

If the total pressure of the gas molecule= 1 aim.

Partial pressure of A= 0.5*1= 0.5 atm

Partial pressure of B= 0.5*1= 0.5 atm

The following videdo might help you to understand the concept.

Questions:

1. CO₂ is added to a container containing 2.5 atm of O₂ to give total pressure 4.0 atm of the gas. What is the partial pressure of CO₂ in the final mixture?

Ans: 1.5 atm