Type any chemical formula and get its molar mass in g/mol, with an element-by-element breakdown showing exactly how the total is built — parentheses, nested groups and hydrates included.
What is molar mass?
Molar mass is the mass of one mole — 6.022 × 10²³ particles — of a substance, in grams per mole (g/mol). It is the bridge between what you weigh on a balance (grams) and what chemical equations count (moles), which makes it the single most-used number in chemistry calculations.
Numerically, molar mass equals the sum of the atomic masses in the formula. One water molecule is about 18 amu; one mole of water is about 18 g.
How to calculate molar mass by hand
Count each element's atoms in the formula (subscripts; parentheses multiply their contents).
Multiply and add: molar mass = Σ (atomic mass × count).
Worked example: water (H₂O)
Find the molar mass of H₂O.
H: 2 × 1.008 = 2.016 g/mol
O: 1 × 15.999 = 15.999 g/mol
Total: 2.016 + 15.999
Answer: ≈ 18.02 g/mol
Worked example: glucose (C₆H₁₂O₆)
Find the molar mass of glucose.
C: 6 × 12.011 = 72.066
H: 12 × 1.008 = 12.096
O: 6 × 15.999 = 95.994
Add the three values
Answer: ≈ 180.16 g/mol
Worked example: aluminium sulfate, Al₂(SO₄)₃
Find the molar mass of Al₂(SO₄)₃ — watch the parentheses.
The subscript 3 multiplies the whole (SO₄) group: 3 S and 12 O.
Al: 2 × 26.982 = 53.964
S: 3 × 32.06 = 96.18
O: 12 × 15.999 = 191.99
Answer: ≈ 342.13 g/mol
Worked example: calcium carbonate (CaCO₃)
Find the molar mass of CaCO₃ — the classic limestone formula.
Ca: 40.078
C: 12.011
O: 3 × 15.999 = 47.997
Add them
Answer: ≈ 100.09 g/mol
Atomic mass vs molar mass
Atomic mass (amu) describes one atom; molar mass (g/mol) describes a mole of atoms or molecules. The numbers match — that's the definition of the mole — but the units and meaning differ. In calculations always carry g/mol so your units cancel correctly.
Common mistakes
⚠ Common mistake: Forgetting that a subscript after a parenthesis multiplies everything inside: Ca(OH)₂ contains 2 O and 2 H, and Al₂(SO₄)₃ contains 12 O — not 4.
⚠ Common mistake: Case matters in symbols: CO is carbon monoxide, but Co is cobalt. Writing “co” or “CO” for cobalt gives a very wrong molar mass.
⚠ Common mistake: Hydrates: the ·5H₂O in CuSO₄·5H₂O adds five whole water molecules (≈ 90 g/mol) — skipping it underestimates the mass by more than a third.
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Frequently asked questions
▸What is the molar mass of water (H2O)?
About 18.02 g/mol: two hydrogens (2 × 1.008) plus one oxygen (15.999).
▸Does this calculator handle hydrates like CuSO4·5H2O?
Yes — use ·, *, or a plain dot between the parts. CuSO4·5H2O gives about 249.7 g/mol.
▸What's the difference between molar mass and molecular weight?
They are numerically the same; “molecular weight” is the older term. Molar mass in g/mol is the modern standard for calculations.
▸Why does my textbook show a slightly different value?
Atomic masses are periodically re-measured and rounded differently between tables. Differences in the second decimal place are normal and don't affect classwork.