Counting atoms by weighing them
A mole is a count — 6.022×10²³ particles, defined so that one mole of an element weighs that element’s atomic mass in grams. The mole is the bridge between the lab balance and what is happening at the atomic scale.
The mole
One mole = 6.022×10²³ particles — Avogadro’s number, denoted Nₐ. Same idea as a "dozen" but enormous: a mole of dollar coins would cover the surface of the Earth several kilometres deep.
Molar mass
The mass of one mole of a substance, in grams per mole. Carbon-12 is exactly 12 g/mol. Add up atomic masses of every atom in the formula. H₂O = 2(1.008) + 16.00 = 18.02 g/mol.
Stoichiometry workflow
Start with a balanced equation. Convert grams → moles using molar mass. Use coefficients as a mole ratio to find moles of another species. Convert back to grams (or particles, or volume).
Limiting reagent & yield
Only one reactant runs out first — the limiting reagent. The others are in excess. Theoretical yield is what stoichiometry predicts; percent yield = (actual / theoretical) × 100. Real yields rarely hit 100%.
Worked example
How many grams of water are produced when 4.0 g H₂ react fully with O₂?
2 H₂ + O₂ → 2 H₂O
- Moles H₂ = 4.0 g ÷ 2.016 g/mol = 1.984 mol
- Mole ratio H₂ : H₂O = 2 : 2, so mol H₂O = 1.984
- Mass H₂O = 1.984 mol × 18.02 g/mol = 35.7 g
Quick formulas
Hands-on tools
Calculate molar masses, convert moles to anything, find the limiting reagent and the yield.
Molar mass calculator
Type any formula, including parentheses and hydrates (use a dot like CuSO4.5H2O).
Examples to try:
Mole ↔ mass ↔ particles ↔ gas volume
Edit any field — the others update automatically (gas volume assumes STP, 22.4 L/mol).
Limiting reagent solver
Enter an equation (balanced or not — we’ll balance it for you) and how many grams of each reactant you start with. The solver names the limiting reagent and predicts the maximum product.
Percent yield
Enter the actual mass produced and the theoretical mass.
Quiz
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Daily challenge
Five problems, one shot. Same set worldwide, refreshes at midnight UTC.
For teachers
Print-ready worksheet, answer key, teaching tips and standards alignment.
Teaching tips
Standards alignment
Reference
Atomic masses
Glossary
Photo gallery
Images sourced from Wikipedia.