How do you calculate the number of moles?

Geschrieben und geprüft von Studoli-Lehrkräften · Aktualisiert am 10. Oktober 2026

Kurze Antwort

Number of moles = mass ÷ molar mass, written n = m ÷ M. Use the mass in grams and the molar mass in g/mol, which you get by adding the relative atomic masses from the periodic table. For example, 36 g of water (M = 18 g/mol) is 36 ÷ 18 = 2 mol.

Schritt für Schritt

Write the formula of the substance.
Find its molar mass by adding the relative atomic masses of every atom in the formula.
Make sure the mass is in grams. Convert kilograms by multiplying by 1000.
Divide the mass by the molar mass to get moles.

Beispiel

How many moles are in 10 g of calcium carbonate, CaCO₃?

M = 40 + 12 + (3 × 16) = 100 g/mol
n = 10 ÷ 100
n = 0.1 mol

Typische Fehler

Forgetting to multiply atomic mass by the small number in the formula, for example O₃.
Using the atomic number instead of the relative atomic mass.
Mixing up the rearrangements. Mass = moles × molar mass.

Nützlich für:GCSE A Level WAEC Matric Bac

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