Molar Mass

Find the molar mass of a compound from its formula (H2SO4, Ca(OH)2, CuSO4·5H2O) with a step-by-step solution and mass fractions.

Write formulas in Latin letters starting with a capital: H2SO4, Ca(OH)2, CuSO4·5H2O (water of crystallization after a dot or *).

Test yourself

The molar mass calculator finds the molar mass of a substance from its chemical formula and shows the solution: the composition, the atomic masses of the elements, the sum and the contribution of each element. Formulas may contain brackets and water of crystallization: Ca(OH)2, Al2(SO4)3, CuSO4·5H2O.

How to find the molar mass

The molar mass M is the mass of one mole of a substance, g/mol. It equals the relative molecular mass M_r numerically — the sum of the relative atomic masses of all atoms in the formula:

M = n₁ · A_r1 + n₂ · A_r2 + …

Here n is the number of atoms of an element (its subscript times the subscript after a bracket), and A_r is the atomic mass from the periodic table.

Atomic masses in school problems

School problems round atomic masses to whole numbers: H = 1, C = 12, N = 14, O = 16, Na = 23, S = 32, Fe = 56. The exception is chlorine: A_r(Cl) = 35.5. For precise work choose “precise: IUPAC Ar” — the values with three to five decimals are used.

Example 1. M(H₂SO₄) = 2 · 1 + 32 + 4 · 16 = 98 g/mol.

Example 2. M(CuSO₄·5H₂O) = 64 + 32 + 9 · 16 + 10 · 1 = 250 g/mol.