Molar MassHow to calculate molar mass, step by step
You need only two things: the formula of the substance and a table of atomic weights. The method is always the same four steps.
- Write the formula clearly.
- Count how many atoms of each element it contains.
- Multiply each count by that element's atomic weight.
- Add the results. The unit is grams per mole.
Example 1: a simple formula
Sulphuric acid is H2SO4. The small numbers tell you the count of the atom just before them. No number means one.
| Element | Atoms | Atomic weight | Total |
|---|---|---|---|
| Hydrogen | 2 | 1.0080 | 2.016 |
| Sulphur | 1 | 32.06 | 32.060 |
| Oxygen | 4 | 15.999 | 63.996 |
Add the last column: 98.072. Rounded sensibly, the molar mass of sulphuric acid is 98.07 g/mol.
Example 2: a formula with brackets
Calcium hydroxide is Ca(OH)2. A number after a bracket multiplies everything inside the bracket. So there are 2 oxygen atoms and 2 hydrogen atoms, and 1 calcium atom.
40.078 + 2 × 15.999 + 2 × 1.0080 = 74.092 g/mol
Aluminium sulphate, Al2(SO4)3, needs more care. The bracket holds one sulphur and four oxygen, and there are three brackets: 3 sulphur and 12 oxygen. With 2 aluminium atoms the total is 342.13 g/mol.
Example 3: water of crystallisation
Blue copper sulphate crystals are CuSO4·5H2O. The dot does not mean multiply. It means "together with". Work out the salt, work out five waters, and add.
159.602 + 5 × 18.015 = 249.677 g/mol
More on these compounds is on the hydrates page.
How many decimal places?
School work in India usually uses whole-number atomic masses: H = 1, C = 12, N = 14, O = 16, Na = 23, S = 32, with Cl = 35.5 and Cu = 63.5 as the common exceptions. With those, sulphuric acid comes to exactly 98. This site uses the fuller values published by IUPAC, so it gives 98.07. Both are right for their purpose. In an exam, use the values printed in the question paper or your textbook, and do not round until the last step.
Check your answer
- Is the answer bigger than the heaviest single atom in the formula? It must be.
- Did you multiply everything inside each bracket?
- Did you read capital letters correctly? Co is cobalt; CO is carbon and oxygen.
- For gases such as oxygen, nitrogen, hydrogen and chlorine, did you use the two-atom molecule, O2 and not O?
Then test it with the calculator, which shows the same table of working.
Which values? This site uses the abridged standard atomic weights published by IUPAC (CIAAW), read on 12 November 2026. Your textbook or question paper may round them differently, for example H = 1 and O = 16, so its answer can differ slightly. In an exam, use the values the paper gives.