Relative formula mass warm-up
Mr is the sum of the Ar values in the formula — nothing more. The two places it goes wrong: brackets multiply everything inside them, and hydrated salts (·nH2O) add n whole water units. Every Ar you need is quoted in the question, A-Level data-sheet style.
Relative formula mass is a GCSE skill — the fuller treatment is in C3 — Mass, equations & measurement.
Mr of O2. Ar: O 16.0.
Mr of NaCl. Ar: Na 23.0, Cl 35.5.
Mr of MgCl2. Ar: Mg 24.3, Cl 35.5.
Mr of Ba(OH)2. Ar: Ba 137.3, O 16.0, H 1.0.
Mr of Na2CO3. Ar: Na 23.0, C 12.0, O 16.0.
Mr of NH4NO3. Ar: N 14.0, H 1.0, O 16.0.
Mr of Fe(NO3)3. Ar: Fe 55.8, N 14.0, O 16.0.
Mr of Al2(SO4)3. Ar: Al 27.0, S 32.1, O 16.0.
Mr of K2CO3. Ar: K 39.1, C 12.0, O 16.0.
Mr of MgSO4·7H2O. Ar: Mg 24.3, S 32.1, O 16.0, H 1.0.
Moles ↔ mass ↔ particles
Two conversions, one constant. n = m ÷ Mr turns mass into moles; one mole = 6.02 × 1023 particles (the Avogadro constant, L), so particles = n × L. The ladder below climbs from one-step to two-step — the two-step questions always go through moles, never straight from grams to particles.
Moles and reacting-mass calculations began at GCSE — see C3 — Moles & reacting masses.
n = mMr · particles = n × 6.02 × 1023
Worked example — molecules in 7.2 g of water (Mr 18.0)
- Grams → moles first:
n = 7.2 ÷ 18.0 = 0.40 mol
- Moles → molecules:
0.40 × 6.02 × 1023 = 2.41 × 1023
Moles in 9.0 g of H2O? Mr 18.0.
Moles in 14.7 g of H2SO4? Mr 98.1. Give 3 s.f.
Mass of 0.25 mol of NaCl? Mr 58.5. Give 3 s.f.
Mass of 2.50 mol of Fe2O3? Mr 159.6.
Molecules in 2.0 mol of CO2? Type e.g. 1.2e24 or 1.2×10^24.
Moles equal to 9.03 × 1022 particles?
Molecules in 3.6 g of H2O? Mr 18.0. Two steps.
Worked steps
- n = 3.6 ÷ 18.0 = 0.20 mol
- molecules = 0.20 × 6.02 × 1023 = 1.20 × 1023
Mass of 4.515 × 1023 molecules of CO2? Mr 44.0. Two steps.
Worked steps
- n = 4.515 × 1023 ÷ 6.02 × 1023 = 0.75 mol
- m = 0.75 × 44.0 = 33.0 g
The Mole Maze
One classic from the classroom, rebuilt. Six conversions stand between you and the exit. A wrong turn is a dead end — it greys out, so just pick another tile and carry on.
How many moles in 4.0 g of NaOH? (Mr 40.0)
What is the mass of 0.20 mol of CaCO3? (Mr 100.1)
How many molecules in 0.50 mol of water?
How many moles in 15.9 g of CuO? (Mr 79.5)
What is the mass of 1.505 × 1023 formula units of NaCl? (Mr 58.5)
How many molecules in 8.8 g of CO2? (Mr 44.0)
Found this harder than expected? That's fixable before September — I take on a small number of A-Level students each year →
How did you do?
Work through the questions above and your score appears here.
Moles never leave you at A-Level — titrations, gases, energetics and equilibria all trade in them. Next: concentration, the version of moles your first titration depends on.
That's fixable before September — I take on a small number of A-Level students each year →