For the reaction-rate questions, revisit enzymes and reaction concepts. Connect calculations to energy transfer in biological systems and review laboratory safety before practical work.
Independent practice • Moles, concentration and reactions
Work through small numerical problems and reaction concepts. The quantities and conditions are stated so you can justify each answer.
This page contains eight practice questions. They are learning exercises, not official exam questions or a complete syllabus guide.
How to use this set
- Answer all eight questions before opening the solutions.
- For calculations, write the equation, substitute values and include units.
- Check the explanation even when your answer is correct. Record the concept behind each mistake.
Practice questions
Question 1
Using a molar mass of 18 g/mol, how many moles are in 36 g of water?
- 0.5 mol
- 1 mol
- 2 mol
- 18 mol
Show answer and explanation
Answer: C. Amount = mass ÷ molar mass = 36 ÷ 18 = 2 mol.
Question 2
How many molecules are in 0.50 mol of a molecular substance? Use 6.02 × 10²³ mol⁻¹.
- 3.01 × 10²³
- 6.02 × 10²³
- 1.204 × 10²⁴
- 0.50
Show answer and explanation
Answer: A. Multiply amount by the Avogadro constant: 0.50 × 6.02 × 10²³ = 3.01 × 10²³ molecules.
Question 3
What is the concentration of 0.20 mol solute in a final solution volume of 0.50 L?
- 0.10 mol/L
- 0.25 mol/L
- 0.40 mol/L
- 2.5 mol/L
Show answer and explanation
Answer: C. Concentration = amount ÷ final solution volume = 0.20 ÷ 0.50 = 0.40 mol/L.
Question 4
What is the approximate molar mass of CO₂, using C = 12 and O = 16 g/mol?
- 28 g/mol
- 32 g/mol
- 40 g/mol
- 44 g/mol
Show answer and explanation
Answer: D. One carbon and two oxygen atoms contribute: 12 + 2 × 16 = 44 g/mol.
Question 5
For 2H₂ + O₂ → 2H₂O, 4 mol H₂ reacts completely with excess O₂. How much water forms?
- 1 mol
- 2 mol
- 4 mol
- 8 mol
Show answer and explanation
Answer: C. The coefficients give a 2:2, or 1:1, mole ratio between hydrogen and water. Therefore 4 mol H₂ gives 4 mol H₂O.
Question 6
Why does increasing temperature often increase a reaction rate?
- Every collision becomes successful.
- More particles can overcome the activation barrier.
- The reaction no longer needs collisions.
- All reactions become irreversible.
Show answer and explanation
Answer: B. A higher temperature changes the energy distribution. More colliding particles can have enough energy to react; suitable orientation still matters.
Question 7
Which collision is most likely to produce products?
- Any collision, regardless of energy
- One with enough energy and suitable orientation
- One with zero kinetic energy
- One between products only
Show answer and explanation
Answer: B. Collision theory requires appropriate contact and sufficient energy for the reaction pathway.
Question 8
A reaction has many collisions but very few produce products. Which explanation is plausible?
- All collisions must react.
- Particles have stopped moving.
- Most collisions have insufficient energy or unsuitable orientation.
- Molar mass is always zero.
Show answer and explanation
Answer: C. Collision frequency alone does not determine success; the activation barrier and orientation also affect the outcome.
Review your work
Score one point for each correct answer. Use the score to identify what to revise, rather than to predict an examination result. Rework any missed question without looking at the solution, then explain your reasoning aloud.
For exam preparation, compare your topic list with the current syllabus issued by your examination authority. This short set makes no claim of full syllabus coverage.
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