Mole calculator

Enter what you know and get everything else: mass, moles, number of particles, and gas volume. Each answer comes with the formula it used, the rearrangement, and the substitution written out.

What do you already know?

Mass (g)

Molar mass (g/mol)

Not sure? Work it out on the molar mass calculator.

Gas conditions your syllabus uses

These genuinely differ between syllabuses, and using the wrong one is a common way to lose marks. Check your data sheet and pick the matching one. It only affects the gas volume answer.

What a mole actually is

A mole is a counting word. Just as a dozen means 12 of something, a mole means 6.022 × 10²³ of something. That number is called Avogadro's constant, and it is huge because atoms are tiny. You need an enormous number of them before you have enough to weigh on a school balance.

That is the whole reason the mole exists. Chemists need to count atoms, but atoms are far too small to count directly. So instead we weigh them, and the mole is the bridge between a mass you can measure and a number of particles you cannot.

The formula everything comes back to: moles = mass ÷ molar mass, usually written n = m ÷ M. Learn that one rearrangement and most mole questions become arithmetic.

The three formulas you need

From mass to moles: n = m ÷ M. Divide the mass in grams by the molar mass in grams per mole. The grams cancel and you are left with moles.

From moles to particles: N = n × N_A, where N_A is 6.022 × 10²³ per mole. Multiply the moles by Avogadro's constant. "Particles" means whatever unit the formula describes: molecules for H₂O, formula units for NaCl, atoms for a pure element like Fe.

From moles to gas volume: V = n × V_m, where V_m is the molar gas volume. This only works for gases, and only at the stated conditions. It also assumes the gas behaves ideally, which is close enough for school questions.

Common questions

Why does my textbook say 22.4 L/mol but my teacher says 24?: They are describing different conditions. 22.4 L/mol is one mole of gas at 0°C and 1 atm, the older definition of standard temperature and pressure. 24.0 L/mol is one mole at RTP, which UK exam boards define as 20°C and 1 atmosphere. This is the value most UK GCSE and IGCSE courses use. IUPAC changed the standard pressure to 100 kPa in 1982, which gives 22.7 L/mol. All three are correct for their own conditions, so match whichever your data sheet states.
Does the gas volume formula work for solids and liquids?: No. One mole of a solid or liquid takes up a completely different volume depending on the substance, because the particles are packed together. The molar gas volume only works because gas particles are so spread out that the substance barely matters.
What counts as one "particle"?: Whatever the formula represents. One mole of H₂O is 6.022 × 10²³ water molecules. One mole of NaCl is 6.022 × 10²³ formula units, not separate ions. One mole of O₂ is 6.022 × 10²³ oxygen molecules, which is 1.204 × 10²⁴ oxygen atoms, since each molecule holds two.
Is Avogadro's constant exactly 6.022 × 10²³?: Since the 2019 redefinition of SI units it has an exact value: 6.02214076 × 10²³ per mole. School courses round it to 6.022 × 10²³, which is what this calculator uses.