Linking Moles, Mass & Mr
Section: 3. Stoichiometry | Syllabus: Cambridge IGCSE Chemistry (0620)
The Mole-Mass Relationship
The mass of one mole of a substance, in grams, is numerically equal to its relative formula mass (Mr). This value is also called the molar mass (units g/mol).
- Mr of carbon (C) = 12, so 1 mole of C has a mass of 12 g (molar mass 12 g/mol)
- Mr of water (H₂O) = 18, so 1 mole of H₂O has a mass of 18 g
- Mr of sodium chloride (NaCl) = 58.5, so 1 mole of NaCl has a mass of 58.5 g
amount of substance (mol) = mass (g) ÷ molar mass (g/mol)
Rearranging the Formula
- Moles (mol) = Mass (g) ÷ Mr
- Mass (g) = Moles (mol) × Mr
- Mr = Mass (g) ÷ Moles (mol)
Cover up the quantity you want to find in the triangle "Mass over Moles × Mr" and the remaining two show you the formula to use.
Cover the quantity you want to find to reveal the correct rearrangement of the formula
Worked Example: Mass from Moles
- Question: calculate the mass of 0.25 mol of calcium carbonate, CaCO₃. (Ar: Ca = 40, C = 12, O = 16) [2]
- Answer: Mr of CaCO₃ = 40 + 12 + (3×16) = 100 [1]; mass = moles × Mr = 0.25 × 100 = 25 g [1]
Worked Example: Moles from Mass
- Question: calculate the number of moles in 49 g of sulfuric acid, H₂SO₄. (Ar: H = 1, S = 32, O = 16) [2]
- Answer: Mr of H₂SO₄ = (2×1) + 32 + (4×16) = 98 [1]; moles = mass ÷ Mr = 49 ÷ 98 = 0.5 mol [1]
Linking to the Avogadro Constant
Mass, moles, and number of particles can all be linked in a single calculation.
Number of particles = (mass ÷ Mr) × Avogadro constant
Worked Example: Molecules from Mass
- Question: how many molecules are there in 88 g of carbon dioxide, CO₂? (Ar: C = 12, O = 16) [2]
- Answer: Mr of CO₂ = 44; moles = 88 ÷ 44 = 2 mol [1]; molecules = 2 × 6.02 × 10²³ = 1.204 × 10²⁴ molecules [1]
Common Mistakes
- ❌ Using mass × Mr to find moles
- ✓ moles = mass ÷ Mr, not mass × Mr - this is the single most common formula-triangle slip
- ❌ Forgetting to calculate Mr first, or forgetting subscripts when doing so
- ✓ always work out Mr as a separate first step, and double check every subscript is counted (e.g. CO₂ has 2 oxygen atoms, not 1)
Interactive revision notes, videos and practice questions load below.