Nobody can count 1023 atoms, but anyone can weigh them. The mole links mass, number of particles and, for gases, volume. Extended content.
One mole of a substance contains 6.02 × 1023 particles (the Avogadro constant). The mass of one mole in grams equals its Ar or Mr, so moles = mass ÷ Mr. One mole of any gas occupies 24 dm3 at room temperature and pressure (r.t.p.), so moles of gas = volume in dm3 ÷ 24, or volume in cm3 ÷ 24 000.
Extended content. Pick a substance in the simulator and drag the mass slider. Watch the number of moles and the number of particles change together. Try 12 g of carbon, 18 g of water and 58.5 g of sodium chloride and see what they share.
| Substance | Ar or Mr | Mass of 1 mol | Contains |
|---|---|---|---|
| carbon, C | 12 | 12 g | 6.02 × 1023 atoms |
| water, H2O | 18 | 18 g | 6.02 × 1023 molecules |
| sodium chloride, NaCl | 58.5 | 58.5 g | 6.02 × 1023 formula units |
Work every conversion through the mole: divide mass by Mr to get moles; multiply moles by 6.02 × 1023 to get particles.
Read whether the question wants molecules or atoms. 9 g of water is 3.01 × 1023 molecules but 9.03 × 1023 atoms, because each H2O molecule contains three atoms.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
The Avogadro constant is:
How many moles are in 88 g of carbon dioxide, CO2? (Mr 44)
What volume does 1 mol of any gas occupy at room temperature and pressure?
What is the mass of 0.5 mol of magnesium atoms? (Ar: Mg 24)
Switch to the gas volumes tab. Put 1 mol of hydrogen, oxygen and carbon dioxide into the syringe in turn: the masses differ but the volume is the same, 24 dm3 at r.t.p.
The 24 dm3 rule is for gases only, at r.t.p. Never use it for a solid, a liquid or a solution. And check the unit: dividing a volume in cm3 by 24 gives an answer 1000 times too big.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
How many moles of gas are in 4.8 dm3 at r.t.p.?
How many molecules are in 9 g of water, H2O? (Mr 18)
What is the mass of 1.2 dm3 of oxygen gas, O2, at r.t.p.? (Ar: O 16)
How many atoms are in 0.5 mol of methane, CH4?
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