Molar mass calculator
Enter your values. The answer and the full worked solution update as you type.
Result
| Element | Atoms | Atomic mass (g/mol) | Mass (g/mol) | Mass percent |
|---|---|---|---|---|
| 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 | 4.037% |
Standard atomic weights (IUPAC conventional values). Books that round them (for example Cl = 35.5) can differ in the last digit.
Work out the molar mass of any compound from its chemical formula. The calculator reads brackets, square brackets and hydrates, counts the atoms of every element, multiplies them by the standard atomic weights and adds them up. It also gives the mass percent of each element, the water of crystallisation of a hydrate and, if you enter a mass, the number of moles and formula units.
Step-by-step solution
Count the atoms of each element
Multiply by the atomic masses and add
Mass percent of each element
Water of crystallisation
Grams to moles
Key formulas
- Moles
- n = m/M
How to calculate molar mass
- Count the atoms of each element in one formula unit. A subscript after a bracket multiplies everything inside it: Ca(OH)2 has 1 Ca, 2 O and 2 H.
- For a hydrate such as CuSO4·5H2O, add the atoms of the water part, multiplied by its coefficient: 5 × (2 H + 1 O).
- Multiply the number of atoms of each element by its atomic mass from the periodic table, in g/mol.
- Add the contributions of all elements. The sum is the molar mass M in g/mol, numerically equal to the formula mass in u.
- For the mass percent of an element, divide its contribution by the molar mass and multiply by 100%.
Questions and answers
What is the difference between molar mass and molecular weight?
Numerically they are the same. The molar mass is the mass of one mole of the substance in g/mol; the molecular (or formula) weight is the mass of one molecule or formula unit in atomic mass units, u. Water has M = 18.015 g/mol and a molecular mass of 18.015 u.
How do I write the formula?
Use element symbols with the correct capitals: Co is cobalt, CO is carbon monoxide. Use round or square brackets for groups, for example Al2(SO4)3 or K3[Fe(CN)6], and a dot or an asterisk for hydrates: CuSO4·5H2O or CuSO4*5H2O.
Which atomic masses does the calculator use?
The IUPAC standard atomic weights, conventional values: H 1.008, C 12.011, N 14.007, O 15.999, Cl 35.45. For elements without stable isotopes it uses the mass number of the longest-lived isotope. Books that round, for example Cl = 35.5, can differ in the last digit.
How do I convert grams to moles?
Divide the mass by the molar mass: n = m/M. For 25 g of CuSO4·5H2O, n = 25/249.68 = 0.100 mol. Multiply by Avogadro’s number, 6.022 × 10²³, for the number of formula units.
What is the mass percent of water in a hydrate?
The mass of the water molecules in one formula unit divided by the molar mass of the hydrate. For CuSO4·5H2O it is 5 × 18.015/249.68 = 36.1%. Heating the hydrate removes this water, which is how the percentage is measured in the lab.
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