How many atoms in a glass of water?

 Given 


 * 1 mole is about 6.022e23 (6 with 23 digits after it) molecules
 * molecular weight of Hydrogen (aka, atomic weight since one atom makes up this molecule) is 1 gram/mole, or 1gm
 * molecular weight of Oxygen is 16gm (Oxygen has 8 protons and 8 neutrons, for a total of 16)
 * water is

Derived


 * 1 cup of water is equal to approximately 236.59 grams of water
 * molecular weight of water is (molecular weight of Hydrogen x 2) + molecular weight of Oxygen = (1gm x 2) + 16gm = 18gm

Calculation


 * 236.59 grams (water) / 18gm (molecular weight of water) = 13.144 moles of water
 * 13.144 moles of water * 6.022e23 molecules per mole = 7.915e24 molecules of water
 * 2/3 water is Hydrogen, so 2/3 * 7.915e24 molecules of water = 5.28e24 atoms of Hydrogen
 * 1/3 water is Oxygen, so 1/3 * 7.915e24 molecules of water = 2.64e24 atoms of Oxygen