Mixtures of Gases

Up to this point, we have focused on pure gases for simplicity.  However, because gases move rapidly with very little contact between individual molecules, they mix homogenously. As a result, in nature we normally encounter mixtures of gases. Air, for example, is about 70% nitrogen, around 18% oxygen with much smaller percentages of argon, carbon dioxide and other trace gases including pollutants.  How do the gas laws apply to mixtures like air?  Since each gas essentially behaves as if it were the only gas in the mixture total pressure must be equal to the sum of the pressures contributed by each of the component gases, or:

                                                          P_total = P₁ + P₂ + P_{3 +}  ....

As we saw earlier in this unit, gas pressure is directly proportional to the number of moles of gas in a volume. Then,the relationship between the fraction of moles of one of the gases multiplied by the total pressure due to all of the gases must give the partial pressure due to the gas of interest:
 

                                                            P₁ =  (moles gas₁ / total moles of gas) *   P_total.

For applications in the atmospheric sciences it is important to remember that water vapor is a gas and contributes to gas pressure in the same way that any other gas would.

Here is a sample problem: