Molar Mass Calculator

How molar mass is calculated

Molar mass is the sum of the atomic masses of all atoms in a formula, weighted by how many of each there are. Each element's symbol is followed by a subscript giving its count — a bare symbol means one atom:

M(Ca(OH)₂) = 40.078 + 2 × (15.999 + 1.008) ≈ 74.092 g/mol

Parentheses group a piece of the formula so a subscript applies to everything inside: Ca(OH)₂ is one calcium, two oxygens and two hydrogens. Hydrates are written with a middle dot — CuSO₄·5H₂O is copper sulfate plus five waters of crystallization, and all of it counts toward the molar mass (249.68 g/mol), because those water molecules are part of what you weigh out of the bottle.

Worked example

Glucose, C₆H₁₂O₆: 6 × 12.011 + 12 × 1.008 + 6 × 15.999 = 72.066 + 12.096 + 95.994 ≈ 180.156 g/mol. That single number is the workhorse of the lab: to make 0.0500 mol you weigh 0.0500 × 180.156 = 9.008 g, and to make 10.0 g you dissolve 10.0 ÷ 180.156 = 0.0555 mol. Sodium chloride (NaCl) is 58.44 g/mol, sulfuric acid (H₂SO₄) is 98.08 g/mol — check them the same way and the calculator shows you the running total element by element.

Molar mass, moles and mass — the one conversion you need

The triangle has three sides: mass (g), moles (n) and molar mass (M, g/mol), with n = m ÷ M and m = n × M. Every stoichiometry problem is a trip around this triangle. Multiply the mole count by Avogadro's number, 6.022 × 10²³, and you get the actual number of molecules — 1 mol of anything, from a grain of sand to a planet, contains the same 6.022 × 10²³ particles, which is one of the more useful facts in all of chemistry.

Frequently asked questions

1. What is molar mass?

The mass of one mole of a substance, in grams per mole (g/mol). Numerically it equals the sum of the relative atomic masses of the atoms in the formula, weighted by their counts.

2. How do you read a subscript like H2O versus parentheses?

A subscript applies to the single element right before it, so H2O has two hydrogen atoms. Parentheses group several atoms so the subscript after them multiplies everything inside, as in Ca(OH)2.

3. What is Avogadro's number and where does it fit?

6.022 × 10²³ particles per mole. Multiplying a mole count by it gives the number of atoms, molecules or ions — so 2 mol of water is 1.204 × 10²⁴ molecules.

4. Does molar mass include water of crystallization?

Yes. A hydrate such as CuSO4·5H2O includes the mass of the five water molecules, giving 249.68 g/mol. The dot separates the salt from its waters of crystallization.