Sunbear, The ideal gas law is not really usefull in this situation. For one thing water is far from an ideal gas. However it would work as a close enough approximation as long as you stayed within the gas phase area of the phase diagram. Also, we’re not talking about a closed system. The pressure is constant at 1 atm and the volume of the water increases about 1000 times from water to steam. The easiest way to find the saturation pressure of water at a certain temp is to use either a steam table or a phase diagram. Once again though, for practical purposes you would apply them for a closed system. It doesn’t work for this system.
“If you stick your finger in a pie, whatever is in the pie will be on your finger, and whatever is on your finger will be in the pie…unless you wear a rubber glove”----some demented old lady
