# How does Dalton's law of partial pressures apply to vapor pressure?

Oct 9, 2016

The vapor pressure of the liquid is ANOTHER partial pressure, that contributes to the overall measured pressure.

#### Explanation:

Mathematically the overall pressure is the sum of the individual partial pressures. And thus for a gaseous mixture:

${P}_{\text{Total}}$ $=$ $\Sigma {p}_{1} + {p}_{2} + {p}_{3.} \ldots .$, where ${p}_{i}$ is an individual partial pressure of a component gas. When gas is collected by bubbling thru water, which is a typical experiment (i.e. the gas from a reaction is bubbled into a water-filled graduated cylinder, where it displaces the water, and thus we can measure a volume of collected gas), the pressure in the graduated cylinder ${P}_{\text{graduated cylinder}}$ $=$ ${P}_{\text{gas"+P_"SVP}}$.

And ${P}_{\text{SVP}}$ is the so-called $\text{saturated vapour pressure}$, this is the vapour pressure of water at a particular temperature. Of course, when the water is at $100$ ""^@C temperature, ${P}_{\text{SVP}} = 1 \cdot a t m$; why? But at temperatures lower than the boiling point, for instance at room temperature, water will still exert a non-zero vapour pressure.

This site reports that the vapour pressure of water is $24$ $m m$ $H g$ at $25$ ""^@C. If you collect a gas at this temperature (and equilibrate it with atmospheric pressure) you MUST substract $24$ $m m$ $H g$ from the measured atmospheric pressure on the day in order to find the pressure exerted by the GAS.