Counting particles by weighing them: moles from mass, the Avogadro constant, masses that react from a balanced equation, and the limiting reactant.
What you will learn
- Convert between mass, moles and Mr
- Use the Avogadro constant, with answers in standard form
- Calculate reacting masses from a balanced equation
- Find the limiting reactant and the mass of product it allows
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 same formula rearranges: mass = moles × Mr, and Mr = mass ÷ moles. So 0.20 mol of NaCl (Mr 58.5) is 0.20 × 58.5 = 11.7 g, and a gas sample of 3.2 g that is 0.050 mol has Mr = 3.2 ÷ 0.050 = 64.
One mole of particles has a mass of Mr grams, so one particle has a mass of Mr ÷ 6.02 × 10²³ grams: one atom of copper (Ar 63.5) is 63.5 ÷ 6.02 × 10²³ = 1.05 × 10⁻²² g.
To count atoms rather than molecules, multiply by the atoms in each molecule: 0.25 mol of CH₄ is 1.51 × 10²³ molecules, and each has 4 H atoms, so it holds 4 × 1.51 × 10²³ = 6.02 × 10²³ H atoms.
The rest of this lesson and its 12 practice questions are locked.
In this lesson
- A mole is a number of particles
- Reacting masses in three steps
- The limiting reactant
- Try it: Find the Ar values
- Try it: Your first mole calculation
12 practice questions, marked as you go; about 25 minutes in all.
Key terms
Mole, Avogadro constant, Moles, Limiting reactant
In this unit: Quantitative chemistry and the mole
Conservation of mass, relative formula mass, percentages and concentrations, the mole, reacting masses, empirical formulae, yield, atom economy, titrations and gas volumes.