MOL; molar mass

The mass of matter (weight) is concentrated for the very biggest part in the nucleons (the protons and neutrons). The electrons weight as good as nothing; you may neglect their mass on the whole.
This also means that the mass of a particle directly depends on the number of nucleons in that particle.
A Hydrogen molecule contains two Hydrogen atoms and one Oxygen atom This way, a new unit was introduces for mass:

the atomic mass unit = the average weight of one nuclear particle

So if a particle contains, for example, hundred nucleons, then the (molecular) mass of this particle will be 100 a.m.u.
One atomic mass unit (a.m.u.) is such an incredible small bit of mass that no balance on earth can really weight it. Balances work with grammes, milligrammes, but not much less than that.
To have one gram of matter (measurable on a balance thus), you need all toghether 6 x 1023 nucleons.
This number is also called: the number of Avogadro), but mostly we talk about: 1 MOL
One MOL in chemistry is a unit of numbers, just like a pair (2), or a dozen (12).
So you can have 1 MOL of molecules, 1 MOL of ions, 1 MOL of electrons, 1 MOL of protons, etcetera.

Don't confuse MOL with the mole in biology!

Check the following:
One water molecule has 18 neucleons; so one water molecule weights 18 a.m.u.
1 MOL of water contains 6 x 1023 water molecules;
1 MOL of water contains 6 x 1023 x 18 nucleons;
6 x 1023 nucleons have a mass of 1 gram → 1 MOL of water weights 18 gram.

  1. the mass of 1 water molecule = 18 a.m.u. = molecular mass (micro level)
  2. the mass of 1 MOL of water molecules = 18 grams = molar mass (macro level)
Molecular mass and molar mass are equal, if you ignore the units.