3.5 The mole and Avogadro's constant
5 exam questions worth 7 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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How many moles are there in 36 g of water? (Mr = 18)
Choose one answer.
Correct Not quite: the answer is 2 mol
Mark scheme
B
Model answer
B: 36 ÷ 18 = 2 mol.
Calculate the mass of 0.20 mol of sodium hydroxide, NaOH (M_\mathrm{r} = 40).
Mark scheme
- 0.20 × 40 = 8.0 g (1)
Model answer
mass = 0.20 × 40 = 8.0 g
Calculate the number of moles in 11 g of carbon dioxide, CO2 (M_\mathrm{r} = 44).
Mark scheme
- 11 ÷ 44 = 0.25 mol (1)
Model answer
moles = 11 ÷ 44 = 0.25 mol
Calculate the number of molecules in 3.2 g of oxygen, O2 (M_\mathrm{r} = 32).
The Avogadro constant is 6.02 × 1023 per mole.
Mark scheme
- moles = 3.2 ÷ 32 = 0.10 mol (1)
- 0.10 × 6.02 × 1023 = 6.02 × 1022 (1)
Model answer
moles = 0.10, so molecules = 0.10 × 6.02 × 1023 = 6.02 × 1022.
Explain why 1 mole of magnesium (A_\mathrm{r} = 24) and 1 mole of carbon (A_\mathrm{r} = 12) contain the same number of atoms but have different masses.
Mark scheme
- one mole of any substance contains 6.02 × 1023 particles (1)
- magnesium atoms are heavier than carbon atoms (twice the mass) (1)
Model answer
A mole always contains 6.02 × 1023 particles, but each magnesium atom has twice the mass of a carbon atom, so a mole of magnesium is 24 g and a mole of carbon is 12 g.
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