How many moles are there in 814.504 g of H2O?

First you need to calculate the molar mass of H2O. To do that, look at the periodic table and add up the AMUs (atomic mass units)

H = 1.01

O = 16.0

1.01 • 2 + 16.0 = 18.02

Next, use stoichiometry to convert grams into moles. To do that, divide 814.504g by the number of grams in one mole of H2O.

814.504 : 18.02 = 45.2 moles

There are 45.2 moles in 814.504 grams of H2O.