Molar Mass Calculator
Molar mass is the mass of one mole of a substance in g/mol, and it has the same number as the relative formula mass (Mr). Type a formula such as H2SO4 or Ca(OH)2 to get its molar mass, with each element's share.
Grams and moles
Learn this properly
Lessons from GCSE Combined and Separate Science on this topic: worked steps, then exam-style questions with new numbers every time.
- Conservation of mass and relative formula massQuantitative chemistry and the moleFree
- Moles, the Avogadro constant and reacting massesQuantitative chemistry and the molePlus
- Atoms, elements, compounds and equationsAtomic structure and the periodic tableFree
See the whole courseThe full course, topic tests and mock papers come with Plus. See Plus
Formulas
| Molar mass | M = the sum of (number of atoms × relative atomic mass) over every element |
|---|---|
| Moles | n = m / M |
Worked example
Find the molar mass of calcium hydroxide, Ca(OH)₂, and the number of moles in 37 g.
- Ca(OH)₂ contains 1 Ca, 2 O and 2 H.
- M = 40.078 + 2 × 15.999 + 2 × 1.008 = 74.092 g/mol.
- n = m / M = 37 / 74.092 = 0.499 mol.
Answer: M ≈ 74.09 g/mol, and 37 g is about 0.50 mol.
Common mistakes
- Multiplying only the first atom after a bracket: in Ca(OH)₂ both the O and the H are doubled.
- Leaving out the water in a hydrate: CuSO₄·5H₂O includes five H₂O.
- Using the mass number instead of the relative atomic mass: chlorine is 35.45, not 35.
- Turning n = m / M upside down: grams divided by g/mol gives moles.
About this tool
This molar mass calculator reads a chemical formula and adds up the atomic masses for you. It handles brackets, square brackets and hydrates, so H2SO4, Ca(OH)2, K4[Fe(CN)6] and CuSO4·5H2O all work. You get the molar mass in g/mol, a table showing each element's share, and the working, plus a converter between grams and moles for mole calculations.
Tips
- A number after a bracket multiplies everything inside it. Ca(OH)2 contains 1 calcium, 2 oxygen and 2 hydrogen atoms, and Mg3(PO4)2 contains 2 phosphorus and 8 oxygen atoms.
- To find moles from a mass, divide by the molar mass: 36.04 g of water (H2O, 18.02 g/mol) is 2 mol.
- Percentage by mass is the element's subtotal divided by the molar mass, times 100. It is the number you need for percentage composition questions.
FAQ
- What is the difference between molar mass and relative formula mass (Mr)?
- They have the same number. Relative formula mass (Mr) has no units: it compares the mass of a formula unit with one twelfth of the mass of a carbon-12 atom. Molar mass is the mass of one mole of the substance, in g/mol. For water, Mr is about 18 and the molar mass is about 18 g/mol.
- How do you work out the number of moles from a mass?
- Divide the mass in grams by the molar mass: n = m / M. For example, 88 g of carbon dioxide (M = 44 g/mol) is 88 ÷ 44 = 2 mol.
- Which atomic masses does it use?
- Standard atomic weights from IUPAC, rounded. Your textbook or data sheet may round some values differently (for example Cl as 35.5), so the last decimal place can differ.
- How do hydrates work?
- The part after the dot is added on, multiplied by its number. CuSO4·5H2O is CuSO4 (159.61 g/mol) plus 5 H2O (90.08 g/mol), which gives 249.69 g/mol.
- Why do I get an error?
- Usually a symbol in the wrong case or an unmatched bracket. The error appears under the box and names the problem, for example an unknown symbol such as Xy or a ( without its ).