If you are collecting  gas by having it bubble into a jar filled with water (which we do) it will be wet. It will pick up 'hitchhiker' water as it passes through. You must subtract the water vapor out. You have a mixture of gas + water, we want to know how much gas was produced. The amount of water vapor present is very sensitive to the temperature. The warmer the water that the gas bubbles through, the more water vapor is present. How to do it: 1. Read problems carefully. Words like wet conditions, over water, bubbled through water mean that you will have to correct for water vapor pressure. 2. Label the variables in the problem. P1, V1, T1, P2, V2, T2.  We are usually solving for V2 in these problems. 3. Get the vapor pressure of water from the chart at the temperature where the water is. Usually this is T2. 4. Subtract this vapor pressure from the pressure where the water is. Usually this is P2. 5. Use the Combined Gas Law to solve for the volume (usually) at the new conditions. 6. The greatest use of this technique is in solving mass-volume stoichiometry problems and then correcting the results for actual laboratory conditions. Vapor pressure chart