Is the periodic table of the elements all of the atoms known to exist?

15.438g H2O

Explanation:

First you need to find the reaction equation:

2H2O+2Na=2NaOH + H2

Hydrogen is a diatomic molecule so it will have a subscript of 2 on the right hand side. From there we can balance the reaction.    

Then we can use stoichiometry:

34.2g NaOH * (1 mol NaOH/39.908g NaOH) * (2 mol H2O/2 mol NaOH) * (18.015g H2O/1 mol H20) = 15.438g H2O

It is important that when you use stoichiometry that all your units cancel out until you only have the unit you want.