Mathematically the overall pressure is the sum of the individual partial pressures. And thus for a gaseous mixture:
P_"Total"PTotal == Sigmap_1+p_2+p_3....., 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_"graduated cylinder" = P_"gas"+P_"SVP".
And P_"SVP" is the so-called "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_"SVP"=1*atm; 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 mm Hg at 25 ""^@C. If you collect a gas at this temperature (and equilibrate it with atmospheric pressure) you MUST substract 24 mm Hg from the measured atmospheric pressure on the day in order to find the pressure exerted by the GAS.
