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As you might have guessed, molecular mass is simply the mass of a molecule. While you may think it's hard to measure the mass of something as small as a molecule, it's a lot easier than it sounds. Since molecules are made of atoms, we can just add up the masses of all the atoms in the molecule, and the sum will be the molecular mass.
To do this addition it helps to know a little something about how much mass an atom has. Atoms are made of protons, neutrons, and electrons. Protons and neutrons have the same mass, so we usually measure the mass of an atom in units of mass that are equal to the mass of one proton, or one neutron. So the mass of an atomic mass of any atom is just the number of protons and neutrons in the atom. But what about the electrons? Electrons are so light that their mass isn't worth counting, and we just ignore them.
Counting the protons and neutrons in an atom is a hard job for a scientist to do, but fortunately for us this job has already been done. The atomic mass for atoms of every element are given on any periodic table.
Since we can easily get the atomic mass of any atom from the periodic table, it is simple to calculate the molecular mass of a molecule. Let's see how this works using water. A water molecule is made up of two hydrogen atoms and one oxygen atom. If you look on your periodic table you'll see that a hydrogen atom has a mass of about 1, and that an oxygen atom has a mass of about 16. So we just add up the masses of two hydrogen atoms and one oxygen atom to get the mass of one water molecule:
Add those numbers up and we can see that the molecular mass of one water molecule is 18. Easy, huh?
As you might be able to figure out, the more atoms there are in a molecule, the higher its molecular mass will be. Polymers are made from giant macromolecules, that contain thousands of atoms. Their molecular masses can sometimes be higher than one million!
For more information, at other Web sites...
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Atomic,
Molecular, and Formula Masses — an informal yet comprehensive tutorial on atomic mass and
its implications for chemistry, created by David Dice, Carlton Comprehensive High School.
Molecular mass — learn how molecular mass can be a lot more complicated for macromolecules than for small molecules, part of The Macrogalleria from the University of Southern Mississippi.