ElementsWhat is molar mass?
The molar mass of a substance is the mass of one mole of it, in grams. One mole is a fixed, very large number of particles: 6.02214076 × 1023, called the Avogadro number. The molar mass in grams per mole has the same number as the mass of one molecule in atomic mass units.
How to work it out by hand
- Write the formula and count the atoms of each element. In Ca(OH)2 there is 1 calcium, 2 oxygen and 2 hydrogen.
- Look up the atomic mass of each element in the periodic table.
- Multiply each atomic mass by the number of atoms.
- Add the results. For Ca(OH)2: 40.08 + 2 × 15.999 + 2 × 1.008 = 74.09 g/mol.
What is it used for?
Molar mass turns grams into moles and back. If a reaction needs 0.5 mole of sodium chloride, and NaCl has a molar mass of 58.44 g/mol, you weigh out 29.22 grams. It is also used to find the percentage of each element in a compound, which this calculator shows as well.
Atomic masses are from PubChem and are rounded. Your textbook may use slightly different rounding, so answers can differ in the last digit.
Mistakes to watch for
- Capital letters. Each element symbol starts with a capital. "CO" is carbon and oxygen, "Co" is cobalt. "NO" is nitrogen and oxygen, "No" is nobelium.
- Brackets. A number after a bracket multiplies everything inside it. In Al2(SO4)3 there are 3 sulfur atoms and 12 oxygen atoms.
- Water of crystallisation. In CuSO4.5H2O the 5 applies to the whole water molecule: 10 hydrogen atoms and 5 oxygen atoms are added.
- Molecules of two atoms. Oxygen gas is O2, so its molar mass is twice the atomic mass of oxygen. The same goes for hydrogen, nitrogen, chlorine and the other gases that go about in pairs.