Brainlag PlusCounting particles by weighing them: moles from mass, the Avogadro constant, masses that react from a balanced equation, and the limiting reactant.
A mole is a number of particles
A mole of any substance contains the same number of particles: 6.02 × 10²³, the Avogadro constant.
One mole has a mass in grams equal to its Mr: 1 mol of water (Mr 18) is 18 g, 1 mol of carbon dioxide (Mr 44) is 44 g.
moles = mass (g) ÷ Mr
n = m ÷ Mr
number of particles = moles × 6.02 × 10²³
All boards: recall these (Higher). Learn the value of the Avogadro constant, although questions often state it.
- How many molecules in 11 g of CO₂? (Mr = 44)
- moles = 11 ÷ 44 = 0.25 mol
- molecules = 0.25 × 6.02 × 10²³ = 1.505 × 10²³ = 1.51 × 10²³ (3 s.f.)
Mass in kg? Change it to grams first. 1.2 kg of a substance with Mr 40 is 1200 ÷ 40 = 30 mol, not 0.03 mol.
The rest of this lesson and its 12 practice questions are in Plus.
Screen 1 of 5, free to read.
- 12 exam-style questions, marked as you go, each with a worked solution
- The Periodic Table beside every question
- Anything you get wrong is saved to your flashcards