Mole and molar mass
A mole is 6.022×10²³ particles (Avogadro's number). Molar mass is the mass of one mole in grams per mole. It equals the atomic or formula mass in atomic mass units. Moles = mass ÷ molar mass.
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Analogy
A mole is like a 'dozen', but for atoms: a counting unit that is huge because atoms are tiny.
Example
Molar mass H₂O = 2(1.008) + 16.00 = 18.02 g/mol18.0 g ÷ 18.02 = 1.00 molMolecules = 1.00 × 6.022×10²³
In practice
Moles let a chemist weigh a powder on a balance and know how many molecules are in it.
Practice
Flashcards
- How many particles are in one mole?6.022×10²³ (Avogadro's number).
Interview questions
- Calculate the mass of NH₃ produced from 28.0 g of N₂ with excess H₂ (N₂ + 3H₂ → 2NH₃).
- Moles N₂ = 28.0 / 28.02 ≈ 1.00
- Ratio 1 N₂ : 2 NH₃
- Moles NH₃ = 2.00
- Mass = 2.00 × 17.03 ≈ 34.1 g
Questions people ask
Why is the mole defined using 6.022×10²³?
The number was chosen so that one mole of atoms has a mass in grams equal to the atomic mass in atomic mass units. For example, one mole of carbon-12 atoms weighs exactly 12 g. This links the atomic scale to masses we can weigh in the lab.
How do I convert grams to number of molecules?
First divide the mass by the molar mass to get moles. Then multiply the moles by Avogadro's number, 6.022×10²³, to get the number of molecules. For 36.04 g of water: 36.04 ÷ 18.02 = 2.000 mol, which is 1.204×10²⁴ molecules.