Vapor Pressure of Water
Graph of vapor pressure of water vs. temperature

Note that when the vapor pressure of the water reaches 1 atmosphere (760 torr) we have reached the Normal Boiling Point of the liquid.

When collecting gases over water, we must compute the "dry" pressure of the collected gas by subtracting the vapor pressure of the water from the total atmospheric pressure in the lab, because:

Ptotal = Pgas + PH2O