How to Calculate Molar Mass

Add up the atoms

Molar mass is the mass of one mole of a substance in grams per mole. It is the sum of the atomic masses of every atom in the formula.

1. Count the atoms of each element. In H₂SO₄ there are 2 H, 1 S and 4 O.
2. Multiply each count by the element's atomic mass from the periodic table: H 1.008, S 32.06, O 15.999.
3. Add them: 2 × 1.008 + 32.06 + 4 × 15.999 = 98.07 g/mol.

Brackets and hydrates

A number after a bracket multiplies everything inside it: Ca(OH)₂ has 1 Ca, 2 O and 2 H. A dot joins water of crystallization, so CuSO₄·5H₂O is copper sulfate plus five whole water molecules, 249.68 g/mol. Weigh out the hydrate using the hydrate's molar mass, not the anhydrous one.

Grams and moles

• Moles = mass ÷ molar mass. 49 g of sulfuric acid is 49 ÷ 98.07 = 0.50 mol.
• Mass = moles × molar mass.

Let the calculator count

Type any formula into the Molar Mass Calculator to get the molar mass and the mass percent of each element. To make a solution from it, use the Molarity Calculator.

More worked examples

• NaCl: 22.990 + 35.45 = 58.44 g/mol.
• Glucose, C₆H₁₂O₆: 6 × 12.011 + 12 × 1.008 + 6 × 15.999 = 72.066 + 12.096 + 95.994 = 180.16 g/mol.

Mass percent

Each element's share of the mass is its total atomic mass divided by the molar mass. In glucose, carbon is 72.066 ÷ 180.16 = 40.0%, hydrogen 6.7% and oxygen 53.3%.

Common mistakes

• Capital letters. Co is cobalt, 58.93 g/mol; CO is carbon monoxide, 28.01 g/mol. Every element symbol starts with a capital and any second letter is lowercase.
• Average and exact masses. Molar mass uses average atomic masses, which include all the natural isotopes. Mass spectrometry reports the monoisotopic mass instead, 180.063 for glucose.
• Slightly different tables. Periodic tables round differently, such as Cl as 35.45 or 35.453. Answers that differ in the second decimal place are usually both right.