The molecular mass of a molecule is just the sum of the masses of each atom in it. So if water has a mass of 18, and hydrogen has a mass of 1, and there are two hydrogens and one oxygen in water (it has formula H2O), than oxygen must weigh 16. That is:
18 - (2*1) = 16
You can also look at a Periodic Table, and it will tell you the mass of every element without doing any calculations! See the Web Links to the left of the answer.
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Atomic mass.
The atomic mass of silver is approximately 107.87 atomic mass units.
The atomic mass of aluminum is approximately 26.98 atomic mass units.
The atomic mass of arsenic is approximately 74.92 atomic mass units.
The atomic mass of potassium is approximately 39.10 atomic mass units.