A sample of H2 with a volume of 35.0 L at STP is reacted with excess O2 to produce H2O. How many grams of H2O are produced?

Question

2 H2 (g) + O2 (g) --> 2 H2O (g)

Answer 1

This is a stoichiometry problem. Here is a worked example. Just follow the steps.
http://www.jiskha.com/science/chemistry/stoichiometry.html

However, when all are gases, one may take a shortcut and use L as if they were moles. Try it, you'll like it.
LH2O = LH2 x (2 moles H2O/2 moles H2) = 35L H2 x (2/2) = 35 L H2O
Then moles H2O = 35/22.4 = ? moles and
grams = moles x molar mass.