Molar Mass Calculator

Formulas

• Molar mass M = the sum of (number of atoms × relative atomic mass) over every element
• Moles n = m / M

Worked example

Find the molar mass of calcium hydroxide, Ca(OH)₂, and the number of moles in 37 g.

1. Ca(OH)₂ contains 1 Ca, 2 O and 2 H.
2. M = 40.078 + 2 × 15.999 + 2 × 1.008 = 74.092 g/mol.
3. n = m / M = 37 / 74.092 = 0.499 mol.

Answer: M ≈ 74.09 g/mol, and 37 g is about 0.50 mol.

Common mistakes

• Multiplying only the first atom after a bracket: in Ca(OH)₂ both the O and the H are doubled.
• Leaving out the water in a hydrate: CuSO₄·5H₂O includes five H₂O.
• Using the mass number instead of the relative atomic mass: chlorine is 35.45, not 35.
• Turning n = m / M upside down: grams divided by g/mol gives moles.

About this tool

This molar mass calculator reads a chemical formula and adds up the atomic masses for you. It handles brackets, square brackets and hydrates, so H2SO4, Ca(OH)2, K4[Fe(CN)6] and CuSO4·5H2O all work. You get the molar mass in g/mol, a table showing each element's share, and the working, plus a converter between grams and moles for mole calculations.

Tips

• A number after a bracket multiplies everything inside it. Ca(OH)2 contains 1 calcium, 2 oxygen and 2 hydrogen atoms, and Mg3(PO4)2 contains 2 phosphorus and 8 oxygen atoms.
• To find moles from a mass, divide by the molar mass: 36.04 g of water (H2O, 18.02 g/mol) is 2 mol.
• Percentage by mass is the element's subtotal divided by the molar mass, times 100. It is the number you need for percentage composition questions.

FAQ

Which atomic masses does it use?

Standard atomic weights from IUPAC, rounded. Your textbook or data sheet may round some values differently (for example Cl as 35.5), so the last decimal place can differ.

How do hydrates work?

The part after the dot is added on, multiplied by its number. CuSO4·5H2O is CuSO4 (159.61 g/mol) plus 5 H2O (90.08 g/mol), which gives 249.69 g/mol.

Why do I get an error?

Usually a symbol in the wrong case or an unmatched bracket. The error appears under the box and names the problem, for example an unknown symbol such as Xy or a ( without its ).