Free chemistry calculator

Molar mass calculator

Type a chemical formula to get its molar mass in g/mol, with every element’s contribution and mass percent shown, so you can check your own work.

Chemical formulaCapitalization matters: Co is cobalt, CO is carbon monoxide. Use ( ) or [ ] for groups and · or . for hydrates.

Molar mass of C₆H₁₂O₆ = 180.156 g/mol

Element	Atoms	Atomic mass	Mass (g/mol)	Mass %
Carbon (C)	6	12.011	6 × 12.011 = 72.066	40.00%
Hydrogen (H)	12	1.008	12 × 1.008 = 12.096	6.71%
Oxygen (O)	6	15.999	6 × 15.999 = 95.994	53.28%
Total			180.156	100.00%

Explain it step by stepUses standard atomic weights. Your class may round them differently (e.g. Cl = 35.5).

How to calculate molar mass

Molar mass is the mass of one mole of a substance, in grams per mole (g/mol). It equals the sum of the atomic masses of every atom in the formula.

Count the atoms of each element. A subscript applies to the element before it, and a subscript after parentheses multiplies everything inside.
Multiply each count by that element’s atomic mass from the periodic table.
Add the results.

Example, Ca(OH)₂: 1 Ca + 2 O + 2 H = 40.078 + 2(15.999) + 2(1.008) = 74.092 g/mol.

Using molar mass to convert grams and moles

Molar mass is the conversion factor between grams and moles:

moles = grams ÷ molar mass
grams = moles × molar mass

Example: 36.0 g of water is 36.0 g ÷ 18.015 g/mol = 2.00 mol H₂O.

For stoichiometry, convert to moles first, use the coefficients from the balanced equation, then convert back to grams. Stuck on a full problem? Ask the chemistry homework solver.

Hydrates and complex formulas

For hydrates like CuSO₄·5H₂O, add the water: 159.60 + 5(18.015) = 249.68 g/mol. Type the dot as · or a period. Square brackets work for coordination compounds such as K₄[Fe(CN)₆].

Mass percent composition

The last column shows each element’s share of the total mass: (element mass ÷ molar mass) × 100. For Fe₂O₃, iron is 111.69 ÷ 159.69 = 69.94% of the mass. That’s the first step for empirical-formula problems.

Questions

Common questions.

What is the difference between molar mass and molecular weight?

They have the same number. Molecular weight (or formula mass) is in atomic mass units per molecule, and molar mass is in grams per mole. Water is 18.015 u per molecule and 18.015 g/mol.

Why is my answer slightly different from my textbook?

Textbooks often round atomic masses, for example Cl = 35.5 or H = 1.01. The calculator uses standard atomic weights, so answers can differ in the last decimal place. Use the values your class uses on tests.

How do parentheses work in a formula?

The subscript after a closing parenthesis multiplies every atom inside. In Al₂(SO₄)₃ there are 3 sulfur atoms and 12 oxygen atoms.

Does capitalization matter?

Yes. Each element symbol starts with a capital letter, so Co is cobalt while CO is one carbon and one oxygen. Type formulas exactly as written.

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