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Moles — counting by weighing
Atoms are too small to count. So we count a heap: 1 mol = 6.02×10²³ particles. Weigh the heap, divide by the molar mass.
Bench tool
The mole tool
Mass ÷ molar mass = moles. Try it:
Moles =
Water is 18 g/mol, NaCl 58.5, CO₂ 44.
How it works
- Moles = mass ÷ molar mass. 18 g of water is 1 mol.
- Molar mass comes from the table. H 1 + Cl 35.5 → HCl 36.5 g/mol.
- One mole is one heap. Same count, different weight per substance.
Worked examples
36 g of water. 36 ÷ 18 = 2 mol.
117 g NaCl. 117 ÷ 58.5 = 2 mol — same count, heavier heap.
How many atoms in 1 mol C? 6.02×10²³ — weighs 12 g.
Where it goes wrong
Grams are not moles. 10 g of H₂ is 5 mol; 10 g of O₂ is 0.31 mol. Convert first.
Try it at home: sugar heap weigh-in
Count the heap. Weigh 1 spoon vs 3 spoons of sugar, then 1 vs 3 spoons of rice — and count the rice grains. Heaps scale exactly: moles are big counted heaps, same count, different weight per substance.
For teachers
Learners compare grams directly — probe by asking which has more particles, 10 g H₂ or 10 g O₂. Quickest check: ask them to convert 36 g of water to moles aloud (36 ÷ 18).