A Level Physics 15.1 The Mole MCQs – 30 Practice Questions with Answers (Cambridge 9702)

This page covers Section 15.1, The Mole, from the Cambridge International AS & A Level Physics (9702) syllabus, an A Level topic in the Ideal Gases unit. Work through 30 original practice questions on amounts of substance, molar mass and the Avogadro constant, and use the Show Answer toggles to check your working.

Formula recap: n = m / M  |  number of particles N = n × NA  |  NA = 6.02 × 1023 mol−1. For the full set of equations for this topic, see our A Level Physics Formula Sheet.

The Mole & Avogadro’s Constant

Q1. How many particles are there in one mole of any substance?

  • (A) 6.02 × 1022
  • (B) 6.02 × 1023
  • (C) 6.02 × 1024
  • (D) It depends on the substance
Show Answer

Answer: B – One mole of any substance always contains the Avogadro constant of particles, NA = 6.02 × 1023, whether the particles are atoms, molecules or ions.

Q2. What is the SI unit of amount of substance?

  • (A) kilogram
  • (B) molecule
  • (C) mole
  • (D) gram per mole
Show Answer

Answer: C – The mole (symbol mol) is the SI base unit for amount of substance. Gram per mole is the unit of molar mass, not of amount.

Q3. What is the molar mass of carbon dioxide, CO2? (C = 12, O = 16)

  • (A) 44 g mol−1
  • (B) 28 g mol−1
  • (C) 40 g mol−1
  • (D) 12 g mol−1
Show Answer

Answer: A – M(CO2) = 12 + (2 × 16) = 44 g mol−1. Each oxygen atom contributes 16 g mol−1.

Q4. What is the molar mass of oxygen gas, O2? (O = 16)

  • (A) 8 g mol−1
  • (B) 16 g mol−1
  • (C) 48 g mol−1
  • (D) 32 g mol−1
Show Answer

Answer: D – An oxygen molecule contains two oxygen atoms, so M(O2) = 2 × 16 = 32 g mol−1.

Q5. The Avogadro constant is best defined as

  • (A) the number of particles in one mole of a substance
  • (B) the mass of one mole of a substance
  • (C) the number of atoms in one gram of carbon
  • (D) the volume occupied by one mole of a gas
Show Answer

Answer: A – The Avogadro constant NA = 6.02 × 1023 mol−1 is the number of particles (atoms, molecules or ions) per mole.

Q6. Why do physicists and chemists count particles in moles rather than individually?

  • (A) Because one mole is always one gram of a substance
  • (B) Because atoms and molecules have no mass
  • (C) Because atoms and molecules are far too small and numerous to count one by one, and the mole links a measurable mass to a known number of particles
  • (D) Because moles can be measured with a balance directly
Show Answer

Answer: C – A single gram of a substance contains around 1022 particles. Counting in moles lets us convert between the mass we can weigh and the number of particles, using N = n × NA.

Q7. What is the molar mass of water, H2O? (H = 1, O = 16)

  • (A) 17 g mol−1
  • (B) 18 g mol−1
  • (C) 20 g mol−1
  • (D) 34 g mol−1
Show Answer

Answer: B – M(H2O) = (2 × 1) + 16 = 18 g mol−1.

Q8. One mole of helium gas contains

  • (A) 4 atoms
  • (B) 1 atom
  • (C) 6.02 × 1023 molecules of He2
  • (D) 6.02 × 1023 atoms
Show Answer

Answer: D – Helium is monatomic, so one mole contains NA = 6.02 × 1023 separate helium atoms, with a total mass of 4 g.

Q9. Which element has a molar mass of 12 g mol−1?

  • (A) Carbon
  • (B) Oxygen
  • (C) Helium
  • (D) Hydrogen
Show Answer

Answer: A – Carbon-12 has a molar mass of exactly 12 g mol−1; the mole is historically defined using carbon-12. Helium is 4, oxygen 16 and hydrogen 1 g mol−1.

Q10. The molar mass of a substance is

  • (A) the mass of one particle of the substance
  • (B) the number of particles in one gram
  • (C) the mass of one mole of the substance
  • (D) the mass of 6.02 × 1023 grams
Show Answer

Answer: C – Molar mass M is the mass of one mole (6.02 × 1023 particles) of the substance, measured in g mol−1 or kg mol−1.

Amounts & Molar Mass Calculations

Q11. How many moles are there in 36 g of water, H2O? (M = 18 g mol−1)

  • (A) 0.50 mol
  • (B) 2.0 mol
  • (C) 18 mol
  • (D) 648 mol
Show Answer

Answer: B – n = m / M = 36 / 18 = 2.0 mol.

Q12. What is the mass of 0.50 mol of carbon dioxide, CO2? (M = 44 g mol−1)

  • (A) 88 g
  • (B) 44 g
  • (C) 11 g
  • (D) 22 g
Show Answer

Answer: D – m = n × M = 0.50 × 44 = 22 g.

Q13. How many moles are there in 8.0 g of helium? (M = 4 g mol−1)

  • (A) 2.0 mol
  • (B) 32 mol
  • (C) 0.50 mol
  • (D) 4.0 mol
Show Answer

Answer: A – n = m / M = 8.0 / 4 = 2.0 mol.

Q14. What is the mass of 3.0 mol of carbon? (M = 12 g mol−1)

  • (A) 4.0 g
  • (B) 12 g
  • (C) 36 g
  • (D) 0.25 g
Show Answer

Answer: C – m = n × M = 3.0 × 12 = 36 g.

Q15. A sample of 32 g of helium (M = 4 g mol−1) and a sample of 32 g of oxygen (M = 32 g mol−1) are compared. Which contains more moles, and by what factor?

  • (A) Oxygen, by a factor of 8
  • (B) Helium, by a factor of 8
  • (C) They contain equal numbers of moles
  • (D) Helium, by a factor of 2
Show Answer

Answer: B – Helium: n = 32 / 4 = 8.0 mol. Oxygen: n = 32 / 32 = 1.0 mol. For equal masses, the substance with the smaller molar mass gives more moles, here 8 times as many.

Q16. How many molecules are there in 9.0 g of water? (M = 18 g mol−1)

  • (A) 3.01 × 1023
  • (B) 6.02 × 1023
  • (C) 1.20 × 1024
  • (D) 3.01 × 1022
Show Answer

Answer: A – First find the amount: n = 9.0 / 18 = 0.50 mol. Then N = n × NA = 0.50 × 6.02 × 1023 = 3.01 × 1023 molecules.

Q17. How many moles are there in 88 g of carbon dioxide? (M = 44 g mol−1)

  • (A) 0.50 mol
  • (B) 1.0 mol
  • (C) 44 mol
  • (D) 2.0 mol
Show Answer

Answer: D – n = m / M = 88 / 44 = 2.0 mol.

Q18. What is the mass of 0.25 mol of oxygen gas, O2? (M = 32 g mol−1)

  • (A) 128 g
  • (B) 32 g
  • (C) 8.0 g
  • (D) 4.0 g
Show Answer

Answer: C – m = n × M = 0.25 × 32 = 8.0 g.

Q19. Which of the following samples contains the greatest amount of substance?

  • (A) 12 g of carbon (M = 12 g mol−1)
  • (B) 8.0 g of helium (M = 4 g mol−1)
  • (C) 18 g of water (M = 18 g mol−1)
  • (D) 44 g of carbon dioxide (M = 44 g mol−1)
Show Answer

Answer: B – Amounts: carbon 12/12 = 1.0 mol; helium 8.0/4 = 2.0 mol; water 18/18 = 1.0 mol; carbon dioxide 44/44 = 1.0 mol. The helium sample contains the most, 2.0 mol.

Q20. A flask contains 0.020 mol of oxygen gas. What is the mass of the gas? (M = 32 g mol−1)

  • (A) 1600 g
  • (B) 32 g
  • (C) 1.6 g
  • (D) 0.64 g
Show Answer

Answer: D – m = n × M = 0.020 × 32 = 0.64 g.


Counting Particles

Q21. How many molecules are there in 2.0 mol of a gas?

  • (A) 3.01 × 1023
  • (B) 6.02 × 1023
  • (C) 6.02 × 1046
  • (D) 1.20 × 1024
Show Answer

Answer: D – N = n × NA = 2.0 × 6.02 × 1023 = 1.20 × 1024 molecules.

Q22. How many atoms are there in total in 1.0 mol of methane, CH4?

  • (A) 6.02 × 1023
  • (B) 3.01 × 1024
  • (C) 2.41 × 1024
  • (D) 5.0 atoms
Show Answer

Answer: B – Each CH4 molecule contains 1 carbon atom and 4 hydrogen atoms, so 1.0 mol of CH4 contains 5.0 mol of atoms in total. N = 5.0 × 6.02 × 1023 = 3.01 × 1024 atoms.

Q23. How many hydrogen atoms are there in 1.0 mol of methane, CH4?

  • (A) 2.41 × 1024
  • (B) 6.02 × 1023
  • (C) 4 atoms
  • (D) 3.01 × 1024
Show Answer

Answer: A – Each molecule has 4 hydrogen atoms, so 1.0 mol of CH4 contains 4.0 mol of hydrogen atoms: N = 4.0 × 6.02 × 1023 = 2.41 × 1024.

Q24. What is the mass of a single water molecule? (M = 18 g mol−1)

  • (A) 18 g
  • (B) 1.1 × 1022 g
  • (C) 3.0 × 10−23 g
  • (D) 6.0 × 10−24 g
Show Answer

Answer: C – Mass of one molecule = M / NA = 18 / (6.02 × 1023) = 2.99 × 10−23 g ≈ 3.0 × 10−23 g.

Q25. How many molecules are there in 44 g of carbon dioxide? (M = 44 g mol−1)

  • (A) 3.01 × 1023
  • (B) 6.02 × 1023
  • (C) 1.20 × 1024
  • (D) 44 × 6.02 × 1023
Show Answer

Answer: B – n = 44 / 44 = 1.0 mol, so N = 1.0 × 6.02 × 1023 = 6.02 × 1023 molecules.

Q26. A container of volume 0.50 m3 contains 3.01 × 1023 gas molecules. What is the number density (number of molecules per unit volume) of the gas?

  • (A) 6.02 × 1023 m−3
  • (B) 1.51 × 1023 m−3
  • (C) 3.01 × 1023 m−3
  • (D) 1.20 × 1024 m−3
Show Answer

Answer: A – Number density = N / V = (3.01 × 1023) / 0.50 = 6.02 × 1023 m−3.

Q27. How many atoms are there in 0.50 mol of oxygen gas, O2?

  • (A) 3.01 × 1023
  • (B) 1.51 × 1023
  • (C) 1.20 × 1024
  • (D) 6.02 × 1023
Show Answer

Answer: D – 0.50 mol of O2 contains 0.50 × 6.02 × 1023 = 3.01 × 1023 molecules. Each molecule has 2 atoms, so the number of atoms is 2 × 3.01 × 1023 = 6.02 × 1023.

Q28. What is the mass of a single helium atom? (M = 4 g mol−1)

  • (A) 4.0 g
  • (B) 2.4 × 1024 g
  • (C) 6.6 × 10−24 g
  • (D) 1.7 × 10−23 g
Show Answer

Answer: C – Mass of one atom = M / NA = 4 / (6.02 × 1023) = 6.6 × 10−24 g.

Q29. How many particles are there in 0.10 mol of a substance?

  • (A) 6.02 × 1023
  • (B) 6.02 × 1022
  • (C) 6.02 × 1024
  • (D) 1.0 × 1022
Show Answer

Answer: B – N = n × NA = 0.10 × 6.02 × 1023 = 6.02 × 1022 particles.

Q30. How many atoms are there in total in 2.0 mol of water, H2O?

  • (A) 3.61 × 1024
  • (B) 1.20 × 1024
  • (C) 6.02 × 1023
  • (D) 6 atoms
Show Answer

Answer: A – Each water molecule contains 3 atoms (2 hydrogen + 1 oxygen), so 2.0 mol of H2O contains 2.0 × 3 = 6.0 mol of atoms. N = 6.0 × 6.02 × 1023 = 3.61 × 1024 atoms.

Keep practising: Explore more A Level Physics topics on our Physics MCQs hub, and keep the key equations for this topic handy with the A Level Physics Formula Sheet.

These are original practice questions written for revision purposes. They are not Cambridge past-paper questions, and this page is not affiliated with or endorsed by Cambridge Assessment International Education.