What molar mass means
Molar mass is the mass of one mole of a substance, measured in grams per mole (g/mol). A mole is a counting unit, like a dozen, except it means 6.022 × 10²³ particles. Molar mass is the bridge between what you can weigh on a balance (grams) and what reactions actually count (moles of atoms and molecules), so you will use it in almost every stoichiometry problem.
Each element's atomic weight on the periodic table is its molar mass in g/mol. Carbon is 12.011 g/mol, so a mole of carbon atoms weighs about 12 grams. For a compound, you add up the atomic weights of every atom in the formula.
The formula
mass % of an element = (atoms × atomic weight) ÷ molar mass × 100
To count atoms, read each subscript as applying to the symbol just before it. A subscript after parentheses or brackets multiplies everything inside. In a hydrate, the number in front of H2O multiplies the entire water molecule.
Example: copper(II) sulfate pentahydrate, CuSO4·5H2O
First count atoms. CuSO4 gives 1 Cu, 1 S and 4 O. Five waters add 5 × 2 = 10 H and 5 × 1 = 5 O, so the total is 1 Cu, 1 S, 9 O and 10 H.
| Element | Atoms × atomic weight | g/mol | Mass % |
|---|---|---|---|
| Cu | 1 × 63.546 | 63.546 | 25.45% |
| S | 1 × 32.06 | 32.06 | 12.84% |
| O | 9 × 15.999 | 143.991 | 57.67% |
| H | 10 × 1.008 | 10.08 | 4.04% |
| Total | 21 atoms | 249.677 | 100% |
So one mole of the blue crystals weighs 249.677 g, and about 36% of that mass (5 × 18.015 = 90.075 g) is water.
Molar masses of common compounds
| Compound | Formula | Molar mass (g/mol) |
|---|---|---|
| Water | H2O | 18.015 |
| Carbon dioxide | CO2 | 44.009 |
| Sodium chloride | NaCl | 58.44 |
| Calcium hydroxide | Ca(OH)2 | 74.092 |
| Sulfuric acid | H2SO4 | 98.072 |
| Glucose | C6H12O6 | 180.156 |
| Iron(III) sulfate | Fe2(SO4)3 | 399.858 |
Common mistakes
- Forgetting to distribute a subscript. In Ca(OH)2 there are 2 hydrogens, not 1. In Fe2(SO4)3 there are 12 oxygens.
- Wrong capitalization. CO (carbon monoxide, 28.010 g/mol) and Co (cobalt, 58.933 g/mol) are very different. Every element symbol starts with a capital letter.
- Leaving out the water in a hydrate. If the label says CuSO4·5H2O, weigh using 249.68 g/mol, not 159.60 g/mol for the anhydrous salt.
- Using atomic number instead of atomic weight. Oxygen's atomic number is 8, but its atomic weight is 15.999.
- Rounding too early. Keep at least the decimals your periodic table gives until the final answer.
Frequently asked questions
What is molar mass?
Molar mass is the mass of one mole of a substance, in grams per mole (g/mol). One mole is 6.022 × 10²³ particles. Numerically, the molar mass in g/mol equals the formula mass in atomic mass units, so water is about 18.015 u per molecule and 18.015 g per mole.
How do I calculate molar mass by hand?
Count the atoms of each element in the formula, multiply each count by that element's atomic weight from the periodic table, and add the results. For water, H2O: 2 × 1.008 + 1 × 15.999 = 18.015 g/mol.
How do parentheses work in a chemical formula?
A subscript after a parenthesis multiplies everything inside it. In Ca(OH)2 the 2 applies to both O and H, giving 1 Ca, 2 O and 2 H. Brackets work the same way and can be nested, as in K4[Fe(CN)6].
How do I enter a hydrate like CuSO4·5H2O?
Use a raised dot, an asterisk or a period between the parts: CuSO4·5H2O, CuSO4*5H2O and CuSO4.5H2O all work. The number in front of H2O multiplies the whole water molecule, so five waters add 10 H and 5 O. Decimal coefficients such as CaSO4·0.5H2O are allowed too.
Why does my textbook give a slightly different answer?
Periodic tables round atomic weights differently. This calculator uses IUPAC's abridged standard atomic weights, such as Cl = 35.45 and S = 32.06. A table that uses Cl = 35.453 or S = 32.065 will give an answer that differs in the second or third decimal place. Use the values your class uses on tests.
Why are some atomic weights whole numbers?
Elements such as technetium, promethium, radon and everything past uranium have no stable isotopes, so there is no natural average to report. By convention the mass number of the longest-lived isotope is used instead, often shown in brackets like [98] for technetium.
Rates and figures last checked October 2026.
This calculator gives estimates for planning. Check current rates and product labels before you buy, list or file.