Chemistry · Molar mass
Molar mass calculator.
Type a formula and get its molar mass in grams per mole, with a line for each element. Brackets and hydrates work.
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How to find molar mass from a formula
Molar mass is the mass in grams of one mole of a substance. Count the atoms of each element in the formula, multiply each count by that element's atomic weight, and add the results. Brackets multiply everything inside them, and a hydrate dot adds the water (or other part) after it, multiplied by the number in front.
Worked example. Ca(OH)2 has 1 Ca, 2 O and 2 H. Ca: 40.078 x 1 = 40.078. O: 15.999 x 2 = 31.998. H: 1.008 x 2 = 2.016. Total: 74.092 g/mol. For the hydrate CuSO4.5H2O the calculator adds five waters to CuSO4 and gets 249.68 g/mol.
The atomic weights are the abridged standard values published by CIAAW (2024), which are quoted to about five significant figures. A different periodic table may round differently and give a third-decimal difference. Elements that have no standard atomic weight, such as technetium and radon, are not in the table.
Frequently asked questions
What is molar mass?
It is the mass in grams of 1 mole of a substance, written in g/mol. For a compound you add the atomic weights of all atoms in the formula.
How do I enter a hydrate such as CuSO4.5H2O?
Type the formula, a dot, the number of waters and H2O, for example CuSO4.5H2O. The dot, a middle dot or an asterisk all work.
Why does my periodic table give a slightly different value?
Tables round atomic weights differently. This tool uses the CIAAW abridged standard atomic weights (2024), quoted to about five significant figures, so results can differ in the last digit from a table that rounds to two decimals.
Why does the tool say an element has no standard atomic weight?
Elements such as technetium, promethium and radon have no stable isotopes and CIAAW does not publish a standard atomic weight for them. Use the mass number of the isotope you have if you need an estimate.
Sources
Checked
- CIAAW: Abridged Standard Atomic Weights 2024
- OpenStax Chemistry 2e, 3.1 Formula Mass and the Mole Concept
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Lab safety and scope
This is arithmetic, not professional advice. Check every result against your own procedure, the safety data sheet (SDS) for each chemical, and your lab supervisor or instructor before you prepare, mix or dispose of anything. Verify values on the reagent bottle, since purity and hydration state change the real molar mass.