WAEC Chemistry past questions and answers

Checked by the Skolra question team · Last updated 23 October 2026

WAEC Chemistry: 490 objective past questions from 10 papers between 2015 and 2024, each with the answer and an explanation. Here are ten.

How the WAEC Chemistry paper is set. On WAEC's 2026 timetable for private candidates, Paper 1 (objective) takes 1 hour, Paper 2 (essay) 2 hours and Paper 3 (alternative to practical work) 1 hour 30 minutes. If the question paper on the day gives another time, follow the question paper. The questions here are from Paper 1.

1. 1.

   An organic compound has the empirical formula $CH_{2}$. If its molar mass is 42$gmol^{-1}$, what is its molecular formula? (C = 12.0, H = 1.0)

   1. A
      C₄H₈
   2. B
      C₂H₄
   3. C
      C₃H₆
      Correct
   4. D
      C₅H₁₀
   Show answerHide answer

   Answer: C

   Empirical formula mass of CH₂: 12 + 2(1) = 14 g mol⁻¹.
   Multiplier n: 42 ÷ 14 = 3.
   Molecular formula: (CH₂)₃ = C₃H₆ (propene or cyclopropane, both with molar mass 42 g mol⁻¹).

2. 2.

   Consider the following half-cell reactions.

   Al_(s) $\to$ Al³⁺_(aq) + 3e^-

   Cu²⁺_(aq) + 2e^- $\to$ Cu_(s)

   The overall equation for the reaction is

   1. A
      2Al_(s) + 3Cu²⁺_(aq) → 2Al³⁺_(aq) + 3Cu_(s)
      Correct
   2. B
      2Al³⁺_(aq) + 3Cu_(s) → 2Al_(s) + 3Cu²⁺_(aq)
   3. C
      Al_(s) + Cu²⁺_(aq) → Al³⁺_(aq) + Cu_(s)
   4. D
      3Al_(s) + 2Cu²⁺_(aq) → 3Al³⁺_(aq) + 2Cu_(s)
   Show answerHide answer

   Answer: A

   To combine the two half-equations, the number of electrons transferred must balance.

   Oxidation half-equation: Al → Al³⁺ + 3e⁻ — multiply by 2 → 2Al → 2Al³⁺ + 6e⁻

   Reduction half-equation: Cu²⁺ + 2e⁻ → Cu — multiply by 3 → 3Cu²⁺ + 6e⁻ → 3Cu

   Adding the two scaled half-equations and cancelling the 6 electrons gives: 2Al_(s) + 3Cu²⁺_(aq) → 2Al³⁺_(aq) + 3Cu_(s).

3. 3.

   consider the reaction represented by the following equation: 2SO$_2$ + O$_2$⇌ 2SO$_3$

   A decrease in the pressure of the reaction would

   1. A
      Shift the equilibrium to the left, favouring the formation of SO₂
      Correct
   2. B
      Shift the equilibrium to the right, favouring the formation of SO₃
   3. C
      Have no effect on the equilibrium position
   4. D
      Increase the rate of the forward reaction only, without shifting the equilibrium
   Show answerHide answer

   Answer: A

   According to Le Chatelier’s principle, decreasing the pressure of a gaseous equilibrium shifts the position of equilibrium towards the side with the greater number of moles of gas. In 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), the left side has 3 moles of gas (2 + 1) and the right side has 2 moles. A decrease in pressure therefore shifts the equilibrium to the left, producing more SO₂ and O₂ at the expense of SO₃. Industrially, high pressure is maintained in the contact process precisely to favour the forward reaction and maximise the yield of sulphur trioxide.

4. 4.

   Which of the following raw materials is used in the plastic industry?

   1. A
      ethene
      Correct
   2. B
      butane
   3. C
      propyne
   4. D
      methane
   Show answerHide answer

   Answer: A

   Ethene (CH₂=CH₂) is the most important raw material in the plastics industry. It is polymerised via addition polymerisation to produce poly(ethene) — polyethylene — the world's most widely used plastic. Other plastics made from ethene derivatives include PVC (from chloroethene) and polystyrene (from phenylethene). Methane, propyne, and butane are not directly polymerised into common industrial plastics.

5. 5.

   The property of a chemical reaction that is affected by the presence of a catalyst is

   1. A
      activation energy
      Correct
   2. B
      equilibrium constant
   3. C
      enthalpy change of the reaction
   4. D
      heat of combustion
   Show answerHide answer

   Answer: A

   A catalyst provides an alternative reaction pathway with a lower activation energy, enabling a greater proportion of collisions to be productive and thus increasing the reaction rate. It does not alter the enthalpy change (ΔH), the position of equilibrium, or the equilibrium constant — only the speed at which equilibrium is attained.

6. 6.

   The first definition of an element was made by

   1. A
      Antoine Lavoisier
   2. B
      John Dalton
   3. C
      Robert Boyle
      Correct
   4. D
      Joseph Proust
   Show answerHide answer

   Answer: C

   Robert Boyle provided the first scientific definition of an element in his 1661 work "The Sceptical Chymist," defining it as a substance that cannot be decomposed into simpler substances by any known chemical means. Lavoisier later compiled the first practical table of elements, Dalton developed atomic theory, and Proust formulated the law of definite proportions.
7. 7.

   Which of the following equimolar solutions has the highest conductivity?

   1. A
      Hydrochloric acid
      Correct
   2. B
      Sodium chloride
   3. C
      Ethanoic acid
   4. D
      Ammonium hydroxide
   Show answerHide answer

   Answer: A

   Conductivity depends on two things: how completely a solute breaks up into ions (degree of ionisation) and how fast those ions can move (ionic mobility).

   Ethanoic acid and ammonium hydroxide are both weak electrolytes — they ionise only slightly in water, so at the same molar concentration they release very few ions. Their conductivity is therefore low.

   Sodium chloride is a strong electrolyte and ionises completely into Na⁺ and Cl⁻, so it conducts better than the two weak ones above. However, the sodium ion moves relatively slowly through water.

8. 8.

   The purity of a solid sample can best be determined by its

   1. A
      Solubility
   2. B
      Density
   3. C
      Melting point
      Correct
   4. D
      Boiling point
   Show answerHide answer

   Answer: C

   A pure solid has a sharp, well-defined melting point. Impurities lower the melting point and widen the melting range: a pure sample melts over less than 1°C, while an impure sample melts over a broader temperature range at a lower temperature. This behaviour makes the melting point the most reliable indicator of purity for a solid substance.

9. 9.

   How many moles are there in 3.0 g of $O_2$? [O = 16.0]

   1. A
      0.30 mol
   2. B
      0.19 mol
   3. C
      10.7 mol
   4. D
      0.094 mol
      Correct
   Show answerHide answer

   Answer: D

   Step 1: Find the molar mass of O₂.
   Since O = 16.0, the molecule O₂ has a molar mass of 2 × 16.0 = 32.0 g/mol.

   Step 2: Apply the mole formula.
   Number of moles = mass ÷ molar mass = 3.0 g ÷ 32.0 g/mol = 0.09375 mol, which rounds to 0.094 mol.

   The option 0.19 mol comes from dividing 3.0 by 16.0 instead of 32.0 — that is, treating O₂ as if it were a single oxygen atom and forgetting that oxygen gas is diatomic.

   The option 10.7 mol comes from flipping the formula and dividing the molar mass by the mass (32.0 ÷ 3.0) instead of mass by molar mass.

10. 10.

    A saturated solution at 30$^0$C will normally produce crystals at a temperature of

    1. A
       Below 30°C
       Correct
    2. B
       Exactly 30°C
    3. C
       50°C or above
    4. D
       Above 30°C
    Show answerHide answer

    Answer: A

    Cooling saturated solution below 30°C causes crystallization due to decreased solubility.

The questions come from past JAMB, WAEC and NECO papers. We use software to help write the answer options and explanations, and every answer is checked twice before it goes on the site. How we check

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