If 500.0 mL of hydrogen are collected over water at 20.0 C and 99.3 kPa what will the volume of the dry gas be at STP?
Use (P1V1/T1) = (P2V2/T2)
For P1 use 99.3 kPa - kPa for water pressure at 20.0 C. Most tables list vapor pressure H2O in mm Hg so I would convert 99.3 kPa to mm Hg first. Vapor pressure vs T for H2O should be in tables in your text or notes. (perhaps in the problem.)