The #"SVP"# is a function of temperature, and is tabulated at various temperatures. Of course, at water's boiling point under standard conditions, the #"SVP"# is #"1 atmosphere"#.
When you collect a gas by bubbling it thru water to a graduated cylinder, this gas is saturated with water vapour.
Thus #P_"laboratory"=P_"gas"+P_"SVP"#.
At #25# #""^@C, "SVP"="23.8 mm Hg".# So you have to subtract this #"SVP"# from the laboratory pressure in order to find #P_"gas"#, the pressure exerted by whatever gas you are collecting.
#"SVP"# is commonly reported in #"mm Hg"# because the mercury manometer is still the chemists' means of measuring pressure. Clearly, you don't need to memorize these values; they should be provided in an exam.
