An AMU is 1.66 times 10 to the -27th kg. Let's try something a little simpler to see this. A water molecule has one O atom and two H atoms. O has 8 protons and 8 neutrons. That's 16 AMU for the O. H has an AMU of one, for one proton. There are two H atoms....giving an AMU of 2 for the H. So, a water molecule has 18 AMU total. The O part is 16/18, or 16 over the total of 18. So the O is 88.88% of the mass, of the water molecule.

The total mass of the molecule is 1.66X10-27 times 18.......which give the mass of one molecule of water in kg. 88.88% of that is the O.....in kg.