100 degrees Celsius. And apparently this answer needs at least 20 characters to explain it well. But yes, the answer is 100 degrees Celsius
You can simply subtract the atomic number from the mass number in order to find the number of neutrons.
5L O2 x 1 mol O2/ 22.4L O2 x 2 mol H20/ 1 mol O2 x 22.4L H20/ 1 mol H20 = 10 L H2O
For future reference though, since its at STP that means that the coefficient are in proportion. Since oxygen has a coefficient of 1 and water has a coefficient of 2 for every 1 liter of oxygen there is 2 liters of water. Hence you started with 5 liters and ended with 10
