The mole: chemistry’s counting unit
The mole is to chemistry what the dozen is to eggs — except instead of 12, it’s 602 sextillion. This absurdly large number exists because atoms are absurdly small: you need that many of them to get a workable, weighable amount of substance.
Every stoichiometric calculation starts or ends with moles. Balanced chemical equations tell you the mole ratio between reactants and products; converting between grams and moles lets you translate those ratios into actual masses you can measure in the lab.
The most common mistake is using the wrong molar mass (atomic mass of an element vs. molecular mass of a compound, or confusing monatomic vs. diatomic forms).
How to Calculate Moles (step by step)
Step One: Identify the molar mass
Determine the molar mass of your substance in g/mol.
For water (H₂O): 2(1.008) + 15.999 = 18.015 g/mol.
Step Two: Divide mass by molar mass
moles = mass (g) ÷ molar mass (g/mol)
For 100 g of water: 100 ÷ 18.015 = 5.551 mol.
Step Three: Convert to particles if needed
molecules = moles × 6.02214076 × 10²³
For 5.551 mol: 5.551 × 6.022 × 10²³ = 3.343 × 10²⁴ molecules.