Hello, Kahlan,
This is an unusual question a a geometry class, but it is one that has an interesting, and useful, answer.
The acronym STP stands for Standard Temperature and Pressure. Gases are sensitive to these conditions, and so a state was defined as a reference point. STP tells us that the data are are 0oC and 1 atmosphere pressure. The surprising fact, to me, is that all gasses occupy a volume of 22.4 moles for every liter of any gas at STP. Any gas. It's a good conversion factor to remember: 22.4 moles/liter.
In this problem, we merely need to look at the units to see how we can use it. We want moles H2O water vapor in a 1 liter container.
(12 liter)*(22.4 moles/liter) = 268.8 moles [270 moles with 2 sig figs]
Bob
