Question

How many atoms are in 131.97 liters of water vapor at STP?

Answer 1

At STP, 1 mol of gas has 22.4 L. First convert to moles using this conversion factor.
Then use Avogadro's number to convert to molecules of water (1 mol = 6.02 x 10^23 particles).
Then, there are 3 atoms in each molecules of water (2 H + 1 O). Use this to convert to atoms.