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Molar Mass: Definition and How to Calculate It

By the Periodixy Editorial Team · Last reviewed July 11, 2026

Molar mass is the mass of one mole (6.022 × 10²³ particles) of a substance, measured in grams per mole (g/mol). To find it, add up the standard atomic masses of every atom in the chemical formula. It is the conversion factor between the grams you weigh on a balance and the moles you use in equations — which makes it the most-used calculation in all of chemistry class.

The good news: calculating it is pure arithmetic. If you can read a formula and use a periodic table, you can find any molar mass.

A laboratory balance used to weigh chemical samples in grams
Photo by cottonbro studio on Pexels

The three-step method

  1. List every element in the formula and count its atoms (watch the subscripts).
  2. Look up each element's atomic mass on the periodic table.
  3. Multiply each atomic mass by its atom count, then add everything up.

Example 1: Water (H₂O)

Find the molar mass of H₂O.

  1. Hydrogen: 2 atoms × 1.008 g/mol = 2.016 g/mol
  2. Oxygen: 1 atom × 15.999 g/mol = 15.999 g/mol
  3. Add: 2.016 + 15.999

Answer: ≈ 18.02 g/mol

Example 2: Glucose (C₆H₁₂O₆)

Find the molar mass of glucose.

  1. Carbon: 6 × 12.011 = 72.066 g/mol
  2. Hydrogen: 12 × 1.008 = 12.096 g/mol
  3. Oxygen: 6 × 15.999 = 95.994 g/mol
  4. Add: 72.066 + 12.096 + 95.994

Answer: ≈ 180.16 g/mol

Example 3: Table salt (NaCl)

What is the molar mass of sodium chloride, NaCl?

  1. Sodium: 1 × 22.990 = 22.990 g/mol
  2. Chlorine: 1 × 35.45 = 35.45 g/mol
  3. Add: 22.990 + 35.45

Answer: ≈ 58.44 g/mol — check it in the molar mass calculator.

Handling parentheses

A subscript after a closing parenthesis multiplies everything inside. In Ca(OH)₂ the “₂” applies to both the O and the H, so the compound contains 1 Ca, 2 O and 2 H.

Example 3: Calcium hydroxide, Ca(OH)₂

Find the molar mass of Ca(OH)₂.

  1. Calcium: 1 × 40.078 = 40.078 g/mol
  2. Oxygen: 2 × 15.999 = 31.998 g/mol
  3. Hydrogen: 2 × 1.008 = 2.016 g/mol
  4. Add them together.

Answer: ≈ 74.09 g/mol

⚠ Common mistake: The most common exam mistake: forgetting that the subscript outside a parenthesis multiplies everything inside. Al₂(SO₄)₃ has 3 sulfur atoms and 12 oxygen atoms — not 1 and 4.

Hydrates: the dot is not multiplication… but acts like it

Hydrates like CuSO₄·5H₂O contain water molecules locked into the crystal. The “·5H₂O” means five extra water molecules per formula unit. Calculate the anhydrous part and the water part separately, then add: CuSO₄ (≈159.6) + 5 × H₂O (≈ 5 × 18.02) ≈ 249.7 g/mol.

Molar mass vs atomic mass vs molecular weight

Atomic mass (in amu) describes a single atom; molar mass (in g/mol) describes a mole of the substance. Numerically they match — one oxygen atom is 15.999 amu, and one mole of oxygen atoms is 15.999 g. “Molecular weight” is an older term for the same idea applied to molecules.

Try the Molar Mass Calculator to check your work — it shows the full element-by-element breakdown, just like these examples.

Summary

  • Molar mass = sum of (atomic mass × atom count) for every element in the formula.
  • Subscripts outside parentheses multiply everything inside.
  • For hydrates, add the water separately: CuSO₄·5H₂O = CuSO₄ + 5 × H₂O.
  • Atomic mass (amu, one atom) and molar mass (g/mol, one mole) share the same number.

Frequently asked questions

What is molar mass?

Molar mass is the mass of one mole of a substance in grams per mole (g/mol). You find it by adding the standard atomic masses of every atom in the formula — for water that is 2 × 1.008 + 15.999 ≈ 18.02 g/mol.

How do you find molar mass?

Count each element's atoms in the formula, multiply each by its atomic mass from the periodic table, and add the results. The molar mass calculator does this and shows the element-by-element breakdown.

What is the molar mass of NaCl?

About 58.44 g/mol: sodium (22.990) plus chlorine (35.45). So one mole of table salt weighs roughly 58.44 grams.

What units does molar mass use?

Grams per mole (g/mol). If a molar mass is 18.02 g/mol, one mole of that substance weighs 18.02 grams.

Why do textbooks give slightly different atomic masses?

Atomic masses are weighted averages of natural isotopes and are periodically re-measured by IUPAC. Differences in the second or third decimal place are normal and rarely matter for classwork.

Is molar mass the same as molecular mass?

Numerically yes, conceptually no: molecular mass is the mass of one molecule in amu; molar mass is the mass of a mole of molecules in g/mol.

Sources & references

  1. Standard Atomic Weights — NIST
  2. Atomic weights of the elements — IUPAC
  3. Sodium chloride (CID 5234) — PubChem (NIH)

Atomic masses and element data follow current IUPAC/NIST standard values. Educational content only — for graded or laboratory work, verify against your course materials. Last reviewed July 11, 2026.

Practise with these tools

Molar Mass CalculatorCalculate the molar mass of any chemical formula with a full element-by-element breakdown and percent composition.Percent Composition CalculatorFind the percent by mass of every element in a compound from its formula.Stoichiometry CalculatorConvert between grams and moles of any two substances in a balanced equation using mole ratios.

Related study guides

How to Read Chemical FormulasDecode any chemical formula: element symbols, subscripts, parentheses, coefficients and hydrate dots — with H2O, Ca(OH)2 and CuSO4·5H2O examples.Stoichiometry Step-by-StepA clear method for stoichiometry: balance the equation, convert to moles, apply the mole ratio, convert back. Includes full worked examples.Atomic Number vs Atomic MassAtomic number counts protons; atomic mass averages isotope masses. Learn the difference, what mass number means, and how to use all three correctly.