Molar Mass Calculator
Type any chemical formula and get its molar mass, also called its molecular weight, in g/mol and broken down element by element. Brackets and hydrate dots are understood, so Ca(OH)2 and CuSO4·5H2O both work.
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What molar mass means
Molar mass is the mass of one mole of a substance, measured in grams per mole. A mole is 6.022 × 1023 particles, so molar mass answers a practical question: if I want that many particles, how many grams do I weigh out?
Molar mass = sum of the atomic masses of every atom in the formula
The convenient part is that you never have to count particles. The atomic mass printed on the periodic table is already scaled so that adding the numbers up gives you grams per mole directly.
How to work it out by hand
Take sulfuric acid, H2SO4.
- Count the atoms of each element: 2 hydrogen, 1 sulfur, 4 oxygen.
- Look up each atomic mass: H is 1.008, S is 32.06, O is 15.999.
- Multiply each mass by its count: 2 × 1.008 = 2.016, 1 × 32.06 = 32.06, 4 × 15.999 = 63.996.
- Add them: 2.016 + 32.06 + 63.996 = 98.07 g/mol.
Brackets multiply everything inside them. In Ca(OH)2 the subscript 2 applies to both the oxygen and the hydrogen, so the compound has one calcium, two oxygen, and two hydrogen.
Hydrates and the dot
Some crystals trap water inside their structure. Copper sulfate pentahydrate is written CuSO4·5H2O, and that dot does not mean multiply. It means "plus five water molecules attached."
So the molar mass is the mass of CuSO4 plus five times the mass of water:
159.61 + (5 × 18.015) = 249.68 g/mol
This matters in the lab. Weighing out 159.61 g of the blue crystals does not give you a mole of copper sulfate, because most of what is on the balance is water. You would need 249.68 g.
Molar mass, molecular mass, and formula mass
| Term | Means | Units |
|---|---|---|
| Molar mass | Mass of one mole of the substance | g/mol |
| Molecular mass | Mass of one molecule, relative to carbon-12 | none, it is a ratio |
| Formula mass | Same idea, used when there are no real molecules | none, it is a ratio |
The numbers are identical, only the units differ. Water has a molecular mass of 18.02 and a molar mass of 18.02 g/mol. "Formula mass" is preferred for ionic compounds like sodium chloride, because a salt crystal is a repeating lattice rather than a set of separate NaCl molecules.
Molar masses you will use often
| Substance | Formula | Molar mass (g/mol) |
|---|---|---|
| Water | H2O | 18.02 |
| Carbon dioxide | CO2 | 44.01 |
| Sodium chloride | NaCl | 58.44 |
| Glucose | C6H12O6 | 180.16 |
| Sulfuric acid | H2SO4 | 98.07 |
| Sodium hydroxide | NaOH | 40.00 |
| Calcium carbonate | CaCO3 | 100.09 |
| Ammonia | NH3 | 17.03 |
| Ethanol | C2H6O | 46.07 |
| Oxygen gas | O2 | 32.00 |
A note on radioactive elements
Thirty four elements have no stable isotope. Technetium, promethium, and everything past bismuth fall into this group. The periodic table shows their value in square brackets, like [98] for technetium.
That bracketed number is not a standard atomic weight. It is the mass number of the most stable known isotope, and it is a whole number rather than a weighted average, because there is no fixed natural mixture of isotopes to average. This calculator uses those values and flags them, so you know the result is an approximation rather than a measured average.
Mistakes worth avoiding
- Case matters. Co is cobalt, CO is carbon monoxide. A capital letter starts a new element.
- Missing the bracket subscript. In Al2(SO4)3 there are three sulfur and twelve oxygen, not one and four.
- Forgetting the water in a hydrate. It is often most of the mass.
- Using 16 for oxygen. The standard value is 15.999. Rounding early builds error into every later step.
- Writing elemental gases as single atoms. Oxygen gas is O2 at 32.00 g/mol, not 16.00.
Frequently asked questions
How do I calculate molar mass?
Count how many atoms of each element the formula contains, multiply each count by that element's atomic mass from the periodic table, then add all the results together. The answer is in grams per mole.
What is the molar mass of water?
Water is H2O, so it is two hydrogen at 1.008 plus one oxygen at 15.999. That gives 2.016 + 15.999, which is 18.02 g/mol.
Is molar mass the same as molecular weight?
The numbers are the same but the units differ. Molar mass is measured in grams per mole. Molecular weight is a ratio compared with carbon-12 and has no units. In everyday use people treat them as interchangeable.
How do I handle a formula with brackets?
The subscript outside the bracket multiplies every atom inside it. In Ca(OH)2 the 2 applies to both the oxygen and the hydrogen, giving one calcium, two oxygen, and two hydrogen atoms.
What does the dot mean in CuSO4·5H2O?
The dot means water molecules are attached to the crystal, not multiplication. CuSO4·5H2O is copper sulfate with five water molecules per formula unit, so its molar mass is 159.61 plus five times 18.015, which is 249.68 g/mol.
Why are some atomic masses in square brackets?
Those elements have no stable isotope, so there is no natural mixture to average. The bracketed figure is the mass number of the most stable isotope known, which makes any molar mass calculated from it an approximation.
Related terms
Next: feed the result into the molarity calculator, or check element data on the interactive periodic table.