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3.2.1Moles

AQA GCSE Chemistry (8462), Higher tier · Quantitative chemistry › Moles and masses

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Calculate the number of moles in 8.8 g of carbon dioxide.
Mr of CO2 = 44
0.2 mol
Chemical amounts are measured in moles.
Relative atomic masses (Ar): C = 12, O = 16 Give the symbol for the unit mole.
mol

Sample questions

Written for this site in the style of AQA exam questions. They are not taken from real past papers.

Question 1Easy5 marks
The number of moles of a substance can be calculated using the equation:
number of moles = mass in g ÷ relative formula mass (Mr)
(a) Calculate the number of moles in 8.8 g of carbon dioxide.
Mr of CO2 = 44[2]
(b) Calculate the mass of 0.25 mol of sodium hydroxide.
Mr of NaOH = 40[2]
(c) Complete the sentence.
The mass of one mole of a substance in grams is numerically equal to its ....................[1]
Show the answer and mark scheme
(a) Answer: 0.2 mol
  • 8.8 ÷ 44
  • 0.2 (mol)
(b) Answer: 10 g
  • 0.25 × 40
  • 10 (g)
(c) Answer: relative formula mass (Mr)
  • relative formula mass / Mr
Question 2Medium7 marks
The Avogadro constant is 6.02 × 1023 per mole.
Relative atomic masses (Ar): C = 12, O = 16, Mg = 24
(a) How many atoms are there in 0.500 mol of magnesium?[1]
(b) Calculate the number of molecules in 13.2 g of carbon dioxide.
Give your answer in standard form to 3 significant figures.[2]
(c) Calculate the mass of 3.01 × 1022 atoms of carbon.[2]
(d) A student said: 'One mole of carbon dioxide contains 6.02 × 1023 atoms.'
Explain why the student is wrong.[2]
Show the answer and mark scheme
(a) Answer: 3.01 × 1023
  • 3.01 × 1023
(b) Answer: 1.81 × 1023
  • moles of CO2 = 13.2 ÷ 44 = 0.3
  • 0.3 × 6.02 × 1023 = 1.81 × 1023
(c) Answer: 0.6 g
  • moles = 3.01 × 1022 ÷ 6.02 × 1023 = 0.05
  • 0.05 × 12 = 0.6 (g)
(d)
  • one mole of CO2 contains 6.02 × 1023 molecules
  • each molecule contains 3 atoms, so there are 3 × 6.02 × 1023 / 1.806 × 1024 atoms
Question 3Hard6 marks
The Avogadro constant is 6.02 × 1023 per mole.
Relative atomic masses (Ar): H = 1, O = 16, S = 32
(a) Calculate the number of oxygen atoms in 4.90 g of sulfuric acid, H2SO4.
Give your answer in standard form to 3 significant figures.[3]
(b) Calculate the mass of sulfuric acid that contains 3.01 × 1023 hydrogen atoms.[3]
Show the answer and mark scheme
(a) Answer: 1.20 × 1023
  • moles of H2SO4 = 4.90 ÷ 98 = 0.05
  • moles of oxygen atoms = 0.05 × 4 = 0.2
  • 0.2 × 6.02 × 1023 = 1.20 × 1023
(b) Answer: 24.5 g
  • moles of H atoms = 3.01 × 1023 ÷ 6.02 × 1023 = 0.5
  • moles of H2SO4 = 0.5 ÷ 2 = 0.25
  • 0.25 × 98 = 24.5 (g)

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