What is the Mole?
The mole is the SI base unit for the amount of a substance, linking the microscopic world of atoms and molecules to measurable, macroscopic quantities like grams. One mole contains exactly 6.022 × 10²³ particles — Avogadro's number.
The number of moles (n) equals the mass of a substance (m) divided by its molar mass (M): n = m/M. Moles let chemists count particles by weighing them.
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Step-by-step worked examples
How many moles are in 36 g of water (M = 18 g/mol)?
n = m/M n = 36/18 n = 2 mol
Find the mass of 0.5 mol of carbon dioxide, CO₂ (M = 44 g/mol).
n = m/M → m = n × M m = 0.5 × 44 m = 22 g
How many moles are in 5.85 g of table salt, NaCl (M = 58.5 g/mol)?
n = m/M n = 5.85/58.5 n = 0.1 mol
Flashcards
Quick quiz
Q1.How many moles are in 44 g of CO₂ (M = 44 g/mol)?
Q2.What is the correct mole formula?
Q3.How many particles are in exactly 1 mole?
Q4.What is the mass of 3 mol of NaCl (M = 58.5 g/mol)?
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Common mistakes
Confusing molar mass with molecular mass units. — Correct: Molar mass is measured in g/mol, while molecular mass (in amu) has the same numeric value but a different unit.
Forgetting to convert mass to grams before using the formula. — Correct: Always convert to grams first — the formula n = m/M requires mass in grams.
Mixing up the formula direction (using M/m instead of m/M). — Correct: Moles = mass ÷ molar mass, so it's always n = m/M, not M/m.
Assuming a mole is a mass unit. — Correct: A mole is a COUNT of particles, not a mass — its mass depends on which substance you're counting.
FAQ
What is the mole concept in chemistry?
The mole is the SI unit for amount of substance, representing exactly 6.022 × 10²³ particles (Avogadro's number).
What is the mole formula?
n = m/M, where n is the number of moles, m is mass in grams, and M is molar mass in g/mol.
How do you calculate the number of moles?
Divide the mass of the substance (in grams) by its molar mass (in g/mol): n = m/M.
What are examples of using the mole formula?
Finding moles in 36 g of water, or finding the mass of 0.5 mol of CO₂, are classic mole formula examples.