Molar Mass Calculator
Type any chemical formula — brackets, complexes and hydrates included — to get its molar mass with full working, element-wise percentage composition and quick gram ↔ mole ↔ particle conversions.
Molar mass is the mass of one mole of a substance in grams per mole, found by adding the atomic masses of every atom in its formula. For sulphuric acid, H₂SO₄ = 2 × 1.008 + 32.06 + 4 × 15.999 = 98.07 g/mol. The number of moles then follows from n = m ÷ M.
Chemical formula
Element symbols are case-sensitive: Co is cobalt, CO is carbon monoxide.
IUPAC standard atomic weights (H = 1.008, C = 12.011, O = 15.999, Cl = 35.45).
Element-wise breakdown
Each element’s share of the molar mass.
| Element | Atoms | Atomic mass (u) | Subtotal (g/mol) | Mass % | |
|---|---|---|---|---|---|
| Total | 100% |
Step-by-step working
Grams ↔ moles ↔ particles
Type in any one box — the other two update. Accepts 6.022e23 or 6.022×10^23.
Molar mass and the mole made easy
Molar mass is the mass of one mole of a substance in grams. It is the sum of the atomic masses of all the atoms in its formula.
Type the formula
Use capitals correctly (NaCl, not nacl). Brackets ( ) [ ] and hydrate dots · . * all work.
Read the molar mass
Get the answer in g/mol with every multiplication and addition shown.
Check the composition
See how much of the mass comes from each element, as a table and a chart.
Convert quantities
Turn grams into moles and particles (or back) using the molar mass.
Molar mass
M = Σ (number of atoms × atomic mass)
H₂SO₄ = 2 × 1.008 + 32.06 + 4 × 15.999 = 98.072 g/mol. With school values (H = 1, S = 32, O = 16) it is 98 g/mol.
The mole concept
n = m ÷ M N = n × NA
NA = 6.022 × 10²³ mol⁻¹ (Avogadro constant). 36 g of water = 36 ÷ 18 = 2 mol = 1.204 × 10²⁴ molecules.
Percentage composition
Mass % = (mass of element in 1 mol ÷ M) × 100
In water, oxygen is 15.999 ÷ 18.015 × 100 = 88.81% of the mass, even though only one of the three atoms is oxygen.
Brackets and hydrates
A number after a bracket multiplies everything inside: Ca(OH)₂ has 2 O and 2 H. A dot joins water of crystallisation: CuSO₄·5H₂O adds 5 × 18.015 = 90.075 to CuSO₄ (159.602), giving 249.677 g/mol.
Molar Mass Calculator — your questions
How do you calculate molar mass?
Multiply the atomic mass of each element by how many of its atoms the formula contains, then add the results. For calcium carbonate, CaCO₃ = 40.08 + 12.011 + 3 × 15.999 = 100.09 g/mol. With the rounded values used in school (Ca = 40, C = 12, O = 16) it comes to 100 g/mol.
What is the molar mass of water?
Water is H₂O, so its molar mass is 2 × 1.008 + 15.999 = 18.015 g/mol, usually written as 18 g/mol in school work. That means 18 g of water is one mole and contains 6.022 × 10²³ water molecules — about 18 mL, roughly a tablespoonful.
How do you convert grams to moles?
Divide the mass by the molar mass: n = m ÷ M. For example, 36 g of water is 36 ÷ 18 = 2 moles. To get the number of particles, multiply the moles by Avogadro's constant: 2 × 6.022 × 10²³ = 1.204 × 10²⁴ molecules.
How do you find the percentage composition of a compound?
Mass percentage of an element = (mass of that element in one mole ÷ molar mass) × 100. In water, oxygen contributes 15.999 out of 18.015, so it makes up 15.999 ÷ 18.015 × 100 = 88.81% of the mass — even though only one of the three atoms is oxygen.
How do you calculate the molar mass of a hydrate like CuSO₄·5H₂O?
Work out the anhydrous salt first, then add the water of crystallisation. CuSO₄ is 159.602 g/mol and 5H₂O adds 5 × 18.015 = 90.075, giving 249.677 g/mol for blue vitriol. A number after a bracket multiplies everything inside, so Ca(OH)₂ contains 2 oxygen and 2 hydrogen atoms.
What is the difference between atomic mass, molecular mass and molar mass?
Atomic mass is the mass of one atom in unified atomic mass units (u). Molecular mass is the sum of the atomic masses in a molecule, also in u. Molar mass is numerically the same number but expressed in grams per mole, because it is the mass of 6.022 × 10²³ such particles.
Why does Co give a different answer from CO?
Element symbols are case-sensitive. Co with a small o is cobalt, a single element of molar mass 58.93 g/mol, while CO with a capital O is carbon monoxide, a compound of molar mass 28.01 g/mol. Always capitalise the first letter of a symbol and keep the second letter lower case.