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bondistry

Molar mass calculator

Type a chemical formula and instantly see its molar mass, each element’s contribution and every step of the calculation.

Examples:

RESULT · M(…)

—g/mol

Type a formula; the result appears here instantly.

What is molar mass?

Molar mass is the mass of one mole of a substance in grams, expressed in g/mol. One mole is 6.02214076 × 10²³ particles (atoms, molecules or formula units), a number known as Avogadro’s constant. Molar mass is the bridge between the grams you can weigh on a balance and the number of particles that actually take part in a reaction.

For an element, the molar mass equals its average atomic mass from the periodic table. Oxygen has an atomic mass of 15.999 u, so one mole of oxygen atoms weighs 15.999 grams. For a compound, the contributions of every atom in the formula are added together.

How to calculate molar mass

First count how many atoms of each element the formula contains. A number after a bracket multiplies every atom inside it: Al2(SO4)3 contains 2 aluminium, 3 sulfur and 3 × 4 = 12 oxygen atoms. In a hydrate, the coefficient after the dot (·) is the number of water molecules; CuSO4·5H2O contains five of them.

Then multiply each element’s atom count by its atomic mass and add up the contributions. Bondistry rounds the result to the precision of the least precise contribution. Because PubChem lists sulfur as 32.07, compounds containing sulfur are shown with two decimal places — no zeros are added that the data does not have.

Mass percent is each element’s contribution divided by the total molar mass. It tells you how much of a compound, by weight, is made of each element, which is handy when checking analytical results.

Converting between grams, moles and particles

Once you know the molar mass, divide a mass by it to get moles: n = m / M. Multiply moles by Avogadro’s constant to get the number of particles: N = n × Nₐ. The converter on this page does both in either direction — type a value in any box.

Worked examples

Example 1

Molar mass of sulfuric acid

What is the molar mass of H2SO4 in g/mol?

  1. 01Count the atoms: 2 hydrogen, 1 sulfur, 4 oxygen.
  2. 02Hydrogen: 2 × 1.0080 = 2.0160
  3. 03Sulfur: 1 × 32.07 = 32.07
  4. 04Oxygen: 4 × 15.999 = 63.996
  5. 05Add them: 2.0160 + 32.07 + 63.996 = 98.0820; sulfur has two decimals, so the result is 98.08 g/mol.

Answer:M(H2SO4) = 98.08 g/mol

Example 2

How many moles are in 50 grams of calcium carbonate?

How many moles and formula units are in 50 g of CaCO3?

  1. 01Molar mass: 40.08 + 12.011 + 3 × 15.999 = 100.088 ≈ 100.09 g/mol.
  2. 02Moles: n = 50 g / 100.09 g/mol = 0.4996 mol.
  3. 03Formula units: 0.4996 × 6.022 × 10²³ = 3.008 × 10²³.

Answer:50 g of CaCO3 ≈ 0.4996 mol ≈ 3.008 × 10²³ formula units