Mathematically the overall pressure is the sum of the individual partial pressures. And thus for a gaseous mixture:
#P_"Total"# #=# #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.
