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n = m ÷ M · N = n × N_A

Mole calculator

Convert between mass, moles and particles, with the molar volume at STP and the formulas shown next to each result.

Short answer: 18.015 g of water is 1 mol, which is 6.02214076 × 10²³ molecules and, if it were a gas at STP, 22.414 litres. Every stoichiometry problem starts from this conversion.

Why moles exist

Reaction equations count particles; balances weigh grams. A mole is a fixed count chosen so the arithmetic lines up: the molar mass in g/mol is numerically the same as the atomic mass in u, so the two units cancel exactly and no conversion factor is invented.

Worked example

5.85 g of sodium chloride (M = 58.44 g/mol) is 0.1001 mol, which is 6.03 × 10²² formula units. In 250 mL of water that is a 0.400 mol/L solution — the same concentration the molarity calculator reports from the other direction.

Mole questions

What is a mole?

Avogadro’s number of particles — 6.02214076 × 10²³. It is the bridge between the mass you weigh on a balance and the number of atoms a reaction equation cares about.

What formulas does it use?

n = m ÷ M to go from grams to moles using the molar mass, and N = n × N_A to go from moles to particles. At STP one mole of an ideal gas occupies 22.414 litres, so that figure is reported too.

Where does the molar mass come from?

It is the sum of the atomic masses in the formula: water H₂O is 2 × 1.008 + 16.00 = 18.015 g/mol. The calculator needs it as an input rather than keeping a table of its own, so the number in the answer is the one you checked.

Is the molar volume always 22.414 litres?

Only for an ideal gas at 0 °C and 1 atm. At room temperature it is closer to 24.5 litres, and for a real gas at high pressure it drifts from either figure — which is why the assumption is stated rather than implied.

Related tools

Molarity calculatorMoles per litre of solution.Ideal gas law calculatorPV = nRT, solved for any term.Combined gas law calculatorP₁V₁/T₁ = P₂V₂/T₂.