Molar Mass Calculator

Any formula, with the working and percent composition.

What molar mass is

The molar mass of a substance is the mass of one mole of it — 6.022 × 10²³ formula units — in grams. Numerically it equals the formula mass in atomic mass units, which is why the periodic table doubles as a molar mass table.

The method

1. Count the atoms of each element, applying any bracket multipliers.
2. Multiply each count by that element's standard atomic weight.
3. Add the subtotals.

Capitalisation is not optional

Element symbols are one capital letter, optionally followed by one lowercase letter. Co is cobalt; CO is carbon monoxide. Typing NACL gives an error rather than a wrong answer, because there is no element Na-C-L reading that is valid.

Brackets and hydrates

A bracket multiplier applies to everything inside it, and nesting works: Al₂(SO₄)₃ contains 2 aluminium, 3 sulfur and 12 oxygen. Hydrates written with a dot, such as CuSO₄·5H₂O, should be entered as CuSO4(H2O)5 here.

What it is used for

Molar mass is the bridge between mass and moles, and therefore between what you can weigh and what the balanced equation tells you. Moles = mass ÷ molar mass. Every stoichiometry problem passes through it.

Percent composition

Each element's subtotal over the total gives its mass share. Run in reverse — percentages to atom ratio — this is how an empirical formula is determined from combustion analysis.