Partial Pressures

Partial Pressure: the partial pressure of a gas is the pressure due to only the molecules of that gas.

For example, suppose we had the following tank , with a total pressure of 1.00 atm:

If we ignore all of the molecules except for nitrogen, the tank would look like this:

Suppose that the pressure from just these nitrogen molecules is 0.45 atm.  This means that the partial pressure of nitrogen is 0.45 atm  We would write:  P_N2 = 0.45 atm.

Dalton’s Law of Partial Pressures: the sum of all the partial pressures in a sealed container equals the total pressure.

P = P₁ + P₂ + P₃ + …

In our tank, this means that P_N2 + P_O2 = 1 atm.

We know (because it was given in the problem) that P_N2 = 0.45 atm

This means P_O2 = 1 atm − 0.45 atm = 0.55 atm

Vapor Pressure (P_v):  every liquid partially evaporates.  If a gas is in a sealed container with a liquid, one of the gases will be the same chemical as the liquid.  The partial pressure of that chemical is called its vapor pressure.

The higher the temperature, the higher the vapor pressure, which means the more of that molecule you’ll have in the gas above the liquid.

normal boiling point:  the temperature when
P_v = P_atmospheric and all of the liquid evaporates.