Molecular weight and molar mass

Molar mass from a formula

Multiply each element's atom count by its atomic weight, then add the contributions to obtain molar mass in g/mol. For H2O, two hydrogen atoms and one oxygen atom give 18.015 g/mol. A number after a bracket multiplies every atom count inside that group.

Entering formulas

Groups such as Ca(OH)2 and K4[Fe(CN)6] are supported. Use · or * for hydrates, as in CuSO4·5H2O. Do not prefix the first formula with a reaction coefficient. The calculation uses abridged standard atomic weights from 2024. Isotopically enriched material, exact ion masses and solids with fractional stoichiometry are outside scope. Displayed digits do not imply measurement accuracy.

Check atom counts first

Verify the formula before examining the digits of the molar mass. Uppercase and lowercase letters have different meanings in element symbols. A subscript applies to the preceding atom, while a number after a closing bracket multiplies the whole group. Enter one compound's formula unit, not an entire reaction equation.

For example, in Ca3(PO4)2 the outer 2 multiplies both phosphorus and oxygen. Calcium count is 3, phosphorus 2 and oxygen 8. Total atoms are 3+2+8 = 13. The molar mass contribution is 3·M(Ca)+2·M(P)+8·M(O). Counting only 4 oxygen atoms changes both formula and result. The atom count output helps identify such input mistakes.

In a hydrate, the number after the separator multiplies the entire water group. Do not confuse it with a reaction coefficient before the first compound. Do not try to bypass an unsupported element or syntax warning by adding spaces. Check the source formula. The result is an average molar mass per formula unit. It does not determine sample purity, mixture composition or isotopic distribution.

Frequently asked questions

How do you calculate the molecular weight of H₂SO₄?

In this example, the formula H2SO4 has a molar mass of 98.072 g/mol and contains 7 atoms per formula unit. Multiply each element’s atomic weight by its atom count and add the contributions, noting that average atomic weights may not represent an isotopically enriched sample.

Calculate with these values