What Is Avogadro's Number and Molar Mass?
Avogadro's number (6.022 × 10²³) is the count of particles in one mole, the bridge between atomic-scale mass and laboratory grams. Molar mass is the mass of one mole, unique to each element or compound.
Avogadro's number is 6.022 × 10²³ particles/mol. Molar mass (M) is the mass in grams of one mole of a substance, numerically equal to its atomic or molecular mass (in u).
- •1 atom of sulfur (S) = 32 u
- •1 atom of oxygen (O) = 16 u
- •1 atom of carbon (C) = 12 u
- •1 mole of S = 32 g/mol
- •1 mole of O = 16 g/mol
- •1 mole of C = 12 g/mol
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Step-by-step worked examples
What is the mass of 2 moles of sodium (Na, molar mass 23 g/mol)?
mass = n × M = 2 mol × 23 g/mol = 46 g
How many moles are in 98 g of sulfuric acid (H₂SO₄, M = 98 g/mol)?
n = mass / M = 98 g / 98 g/mol = 1 mol
How many atoms are in 0.5 mol of carbon?
atoms = n × Nₐ = 0.5 mol × 6.022 × 10²³ = 3.011 × 10²³ atoms
Flashcards
Quick quiz
Q1.Avogadro's number is…
Q2.Molar mass of a compound equals…
Q3.5 moles of O₂ (M = 32 g/mol) has mass…
Q4.Number of atoms in 1 mole of Ne is…
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Common mistakes
Forgetting that atomic mass in u numerically equals molar mass in g/mol. — Correct: Carbon-12 = 12 u/atom = 12 g/mol; this exact relation defines both.
Using Avogadro's number as a simple ratio without units. — Correct: Nₐ = 6.022 × 10²³ particles/mol — the unit 'per mole' is essential.
Confusing atomic mass with molar mass. — Correct: Atomic mass is per atom (u); molar mass is per mole (g/mol).
Not converting moles to atoms correctly. — Correct: atoms = moles × 6.022 × 10²³; check dimensional analysis.
FAQ
What is Avogadro's number?
6.022 × 10²³ particles per mole, named after Amedeo Avogadro.
How do you calculate molar mass?
Sum the atomic masses (in u) of all atoms in the molecule; that's the molar mass in g/mol.
Why is Avogadro's number so large?
Atoms and molecules are incredibly tiny; 6.022 × 10²³ makes their mass measurable in grams.
What is the molar mass of water (H₂O)?
(2 × 1) + 16 = 18 g/mol.