Calcium metal and water produce hydrogen gas. If the volume of wet gas collected over water is 95.0 mL at 20 °C and 758 mm Hg, what is the volume of dry gas at STP conditions? (The vapor pressure of water at 20 °C is 17.5 mm Hg.)

Answer 1

pressure dry gas=pressure wet gas-vaporpressurewater

So you know pressuredrygas

volumegas=95ml
pressuregas=758-17.5
temp: 20C

V2*Pstp/tempstp=volumegas*pressure/temp

where on the right side, it refers to the conditions immediately above.