The formula
M = Σ (atomic mass × number of atoms)
What it means
Molar mass is what one mole weighs, and a mole is 6.022 × 10²³ particles. It comes from adding up the atomic mass of each element as many times as it appears in the formula, so the only table you need is the periodic one. The result is in grams per mole and matches the molecular mass in atomic mass units: water weighs 18.015 u per molecule and 18.015 g per mole.
How to work it out by hand
- Count how many atoms of each element the formula contains
- Look up the atomic mass of each element in the periodic table
- Multiply each atomic mass by its number of atoms
- Add all the products together: that is the molar mass in g/mol
What is worth knowing
Watch the capitals, because they change the compound: "Co" is cobalt and "CO" is carbon monoxide, "Cs" is caesium and "CS" would be carbon with sulfur. Hydrates have their water counted separately, which is why they are written with a middle dot; a plain full stop or an asterisk work here too, since those are the ones on the keyboard. The atomic masses are the IUPAC standard values: they are revised every few years and change from the fourth digit down, so for precision work check the current source.