How many grams of oxygen are collected in a reaction where 245 mL of oxygen gas is collected over water at a temperature of 25 ^\circ {\rm C} and a total pressure of 688 torr

Answer 1

Look up vapor pressure of H2O at 25C and
subtract from 688 torr.
688 torr - vpH2O = pressure dry gas.
Then use PV = nRT and solve for n = number of mols oxygen. Don't forget P must be in atmospheres (torr dry gas/760) and T must be in kelvin. V must be in L.
Then n = grams O2/molar mass O2. Solve for grams.