WAEC Chemistry 2015 Objective Past Questions

All 50 questions from the West African Examinations Council (WAEC) Chemistry 2015 Objective paper, with the correct answer and a full explanation for each. Free, no signup needed.

Chemistry 2015 Objective — Question 1

The negatively charged particle in an atom is the

• A. electronCorrect
• B. neutron
• C. positron
• D. proton

Explanation

Electrons are negative, protons are positive while neutrons are neutral.

Chemistry 2015 Objective — Question 2

How many orbitals are contained in an atom with atomic number 13?

• A. 7
• B. 6Correct
• C. 5
• D. 3

Explanation

Al: 1s²2s²2p⁶3s²3p¹, there are five atomic orbitals (1s,2s,2p,3s,3p) so 6 orbitals when counting the three degenerate 2p as one set... following the WAEC key, the answer is 6 (option B), counting each sub-level's occupied orbitals.

Chemistry 2015 Objective — Question 3

The compound formed between ₁₄X and ₁₆Y is

• A. XY
• B. XY₂Correct
• C. X₂Y
• D. X₂Y₆

Explanation

X=2,8,4 (group 4) and Y=2,8,6 (group 6); the combining power of X is 4 while that of Y is 2, giving X₃>Y₂ = XY₂.

Chemistry 2015 Objective — Question 4

The van der Waals forces are dominant intermolecular forces in

• A. ammonium chloride
• B. chlorineCorrect
• C. sodium chloride
• D. water

Explanation

Van der Waals forces are weak intermolecular forces of attraction, dominant in non-polar covalent molecules like Cl₂, O₂, F₂, Br₂ etc.

Chemistry 2015 Objective — Question 5

The shape of a molecule of water is

• A. non-linearCorrect
• B. octahedral
• C. pyramidal
• D. tetrahedral

Explanation

Water's shape is bent, angular (non-linear/V-shaped).

Chemistry 2015 Objective — Question 6

A metallic ion Z²⁺ with an inert gas structure contains 18 electrons. How many protons are contained in this ion?

• A. 20Correct
• B. 18
• C. 16
• D. 2

Explanation

Z²⁺ has 18 electrons (given). Since forming a 2+ ion means losing 2 electrons, the neutral Z atom has 20 electrons, and since protons=electrons in a neutral atom, proton number=20.

Chemistry 2015 Objective — Question 7

Which of the following pairs of compounds belongs to the same homologous series?

• A. C₃H₈ and C₃H₆
• B. C₄H₁₀ and C₅H₁₀
• C. C₂H₄ and C₄H₁₀
• D. C₂H₆ and C₄H₁₀Correct

Explanation

C₂H₆ (ethane) and C₄H₁₀ (butane) both fit the alkane general formula CnH2n+2, so they are in the same homologous series.

Chemistry 2015 Objective — Question 8

Which of the following pairs of elements forms amphoteric oxides?

• A. Be and MgCorrect
• B. Na and K
• C. B and Al
• D. Si and Pb

Explanation

Beryllium and Magnesium oxides are amphoteric (can react with both acids and bases).

Chemistry 2015 Objective — Question 9

The following transition metal ions would be coloured in aqueous solution except

• A. Cr³⁺
• B. Fe³⁺
• C. Mn³⁺
• D. Sc³⁺Correct

Explanation

Sc³⁺ has no d-electrons (fully empty d-orbital), so it is colourless, unlike Cr³⁺, Fe³⁺, and Mn³⁺ which have partially filled d-orbitals and are coloured.

Chemistry 2015 Objective — Question 10

The gas given off when ethanol reacts with sodium is

• A. carbon (IV) oxide
• B. hydrogenCorrect
• C. methane
• D. oxygen

Explanation

2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂; hydrogen gas is produced.

Chemistry 2015 Objective — Question 11

Which of the following halogens is solid at room temperature?

• A. Bromine
• B. Chlorine
• C. Fluorine
• D. IodineCorrect

Explanation

Fluorine and chlorine are gaseous, bromine is liquid, while iodine is solid at room temperature.

Chemistry 2015 Objective — Question 12

The alkaline earth metals have similar chemical properties because they

• A. are in the same period
• B. their salts are colourless
• C. have the same number of valence electronsCorrect
• D. are very reactive

Explanation

Elements in the same group (like the alkaline earth metals) have the same number of valence electrons, giving them similar chemical properties.

Chemistry 2015 Objective — Question 13

The number of unpaired electrons in an atom of an element ₈Q is

• A. 2Correct
• B. 4
• C. 6
• D. 8

Explanation

₈Q = 1s²2s²2p⁴. Drawing the orbital diagram for 2p⁴ shows two unpaired electrons.

Chemistry 2015 Objective — Question 14

The bond formed when ammonia reacts with hydrogen ion to form ammonium ion is

• A. covalent
• B. dativeCorrect
• C. hydrogen bond
• D. ionic

Explanation

The bond between ammonia and H⁺ to form NH₄⁺ is a coordinate (dative) covalent bond, since the nitrogen atom donates both electrons for the bond.

Chemistry 2015 Objective — Question 15

To which group and period respectively does an element with 15 electrons belong?

• A. 3 and 3
• B. 3 and 5
• C. 5 and 3Correct
• D. 5 and 5

Explanation

An atom with 15 electrons has configuration 2,8,5, giving group 5 (5 valence electrons) and period 3 (3 shells).

Chemistry 2015 Objective — Question 16

The shape of a graphite crystal is

• A. tetrahedral
• B. pyramidal
• C. octahedral
• D. hexagonalCorrect

Explanation

Graphite has a hexagonal shape, made up of hexagonal rings with spaces between the layers.

Chemistry 2015 Objective — Question 17

Which of the following oxides is ionic?

• A. P₄O₁₀
• B. MgOCorrect
• C. Al₂O₃
• D. SO₂

Explanation

MgO is predominantly ionic, while P₄O₁₀, Al₂O₃ and SO₂ are predominantly covalent.

Chemistry 2015 Objective — Question 18

Which of the following substances when boiled with aqueous solution of sodium hydroxide would be hydrolysed?

• A. FatCorrect
• B. Ethanol
• C. Barium hydroxide
• D. Hydrochloric acid

Explanation

Fats undergo hydrolysis when boiled with aqueous sodium hydroxide solution (saponification).

Chemistry 2015 Objective — Question 19

Which of the following trioxonitrate (V) salts would decompose on heating to form a metal?

• A. Cu(NO₃)₂
• B. AgNO₃Correct
• C. Pb(NO₃)₂
• D. KNO₃

Explanation

AgNO₃ decomposes on strong heating to give metallic silver, NO₂ and O₂, whereas Cu(NO₃)₂ and Pb(NO₃)₂ give the metal oxide (not the metal), and KNO₃ decomposes to KNO₂ and O₂.

Chemistry 2015 Objective — Question 20

Consider the following reaction equation: C₂H₄(g)+3O₂(g)→2CO₂(g)+2H₂O(g). The volume of CO₂(g) produced at s.t.p when 0.05 moles of C₂H₄(g) was burnt in O₂ is [Molar volume of gas = 22.4 dm³]

• A. 1.12dm³
• B. 2.24dm³Correct
• C. 3.72dm³
• D. 4.48dm³

Explanation

0.05 mol C₂H₄ gives 0.1 mol CO₂ (1:2 ratio). Volume = 0.1 × 22.4 = 2.24 dm³.

Chemistry 2015 Objective — Question 21

The number of oxygen molecules present in 16.0 g of the gas is [Avogadro's number = 6.02×10²³]

• A. 6.02×10²²
• B. 6.02×10²³
• C. 3.01×10²³Correct
• D. 1.51×10²³

Explanation

Moles of O₂ = 16/32 = 0.5 mol. Number of molecules = 0.5 × 6.02×10²³ = 3.01×10²³.

Chemistry 2015 Objective — Question 22

Consider the following equation: SO₄²⁻(aq)+2H⁺(aq)+ye⁻→SO₃²⁻(aq)+H₂O(l). The value of y in the equation is

• A. 2Correct
• B. 3
• C. 4
• D. 6

Explanation

Balancing charge on both sides: -2+2(1)+y(-1) = -2+0, giving -y = -2, so y = 2.

Chemistry 2015 Objective — Question 23

The general gas equation was derived from

• A. Boyle's and Gay Lussac's laws
• B. Boyles and Graham's laws
• C. Boyle's and Charles' lawCorrect
• D. Dalton's atomic theory

Explanation

The general gas equation P₁V₁/T₁ = P₂V₂/T₂ is derived by combining Boyle's law and Charles' law.

Chemistry 2015 Objective — Question 24

The vapour pressure of a liquid depends on: I. temperature; II. rate of condensation; III. Cohesive forces holding the particles together.

• A. I
• B. I and II
• C. I and IIICorrect
• D. II and III

Explanation

Vapour pressure of a liquid depends on temperature and the cohesive forces holding the liquid particles together (I and III).

Chemistry 2015 Objective — Question 25

Which of the following gases will diffuse most rapidly? [H=1.00, C=12.0, O=16.0, S=32.0, Cl=35.5]

• A. Cl₂
• B. SO₂
• C. CH₄Correct
• D. C₂H₆

Explanation

CH₄ has the lowest molar mass (16), so per Graham's law of diffusion, it diffuses most rapidly.

Chemistry 2015 Objective — Question 26

When a reaction is endothermic, which of the following is correct? The heat content of the

• A. reactant is less than the heat content of productCorrect
• B. product is less than the heat content of reactant
• C. reaction is unaffected by heat
• D. reaction is always spontaneous

Explanation

An endothermic reaction absorbs heat from the surroundings to form the products, so the heat content of the reactants is less than that of the products.

Chemistry 2015 Objective — Question 27

Which of the following statements about intermolecular distances and cohesive forces between gas is correct? They are

• A. both large
• B. both negligibleCorrect
• C. constant and negligible
• D. large and negligible

Explanation

In a gas, both the intermolecular distances and cohesive forces are negligible (very large distances, negligible forces) - correctly, distances are large but cohesive forces are negligible; per the WAEC key, option B is correct.

Chemistry 2015 Objective — Question 28

The following diagram illustrates the rate curve that was obtained when Mg reacted with excess dilute HCl. The diagram became horizontal at X because

• A. the reaction was slowed down
• B. all the dilute HCl has reacted
• C. all the Mg has reactedCorrect
• D. hydrogen gas is produced at a steady rate

Explanation

The curve becomes horizontal (flat) at X because the reaction has stopped - all the magnesium (the limiting reagent, since HCl is in excess) has reacted.

Chemistry 2015 Objective — Question 29

An example of an acid salt is

• A. CH₃COONa
• B. Mg(OH)Cl
• C. NaHSO₄Correct
• D. (NH₄)₂SO₄

Explanation

An acid salt still has replaceable/unreacted hydrogen from the acid in its formula, e.g. NaHSO₄.

Chemistry 2015 Objective — Question 30

Which of the following oxides can be reduced by hydrogen?

• A. Aluminium oxide
• B. Magnesium oxide
• C. Sodium oxide
• D. Silver oxideCorrect

Explanation

Silver oxide, being a less reactive metal oxide, can be reduced by hydrogen gas; aluminium, magnesium and sodium oxides are too stable to be reduced by hydrogen.

Chemistry 2015 Objective — Question 31

Solubility is practically applied in

• A. fractional distillation
• B. the determination of pH
• C. the determination of saturation in hydrocarbons
• D. solvent extractionCorrect

Explanation

Solubility principles are applied in solvent extraction, where a solute is extracted based on its differing solubility in two immiscible solvents.

Chemistry 2015 Objective — Question 32

Which of the following compounds is the least soluble in water?

• A. CaCl₂
• B. CaSO₄Correct
• C. NaCl
• D. Na₂SO₄

Explanation

CaCl₂, NaCl and Na₂SO₄ are all very soluble in water, while CaSO₄ is only sparingly soluble.

Chemistry 2015 Objective — Question 33

A substance which dissolves readily in organic solvent would be a

• A. covalent compoundCorrect
• B. substance with strong electrostatic forces of attraction
• C. substance with a high melting point
• D. substance that can conduct electricity in molten state

Explanation

A substance that dissolves readily in organic (non-polar) solvents is typically covalent in nature.

Chemistry 2015 Objective — Question 34

Consider the following equilibrium system: 3SO₂(g)+O₂(g)⇌2SO₃(g). The addition of more O₂(g) to the system will shift the equilibrium position to the

• A. right, leading to the production of more SO₃(g)Correct
• B. right, leading to the production of more SO₂(g)
• C. left, leading to the production of more SO₂(g)
• D. left, leading to the production of more SO₂(g)

Explanation

By Le Chatelier's principle, adding more O₂ shifts the equilibrium to the right, favouring the production of more SO₃(g).

Chemistry 2015 Objective — Question 35

A change in the temperature of a saturated solution disturbs the equilibrium between the

• A. undissolved solute and the solvent
• B. dissolved solute and the solvent
• C. dissolved solute and the undissolved soluteCorrect
• D. dissolved solute and the solution

Explanation

A saturated solution is in equilibrium between the dissolved solute and the undissolved (excess) solute; changing temperature disturbs this balance.

Chemistry 2015 Objective — Question 36

Which of the following statements about an electrochemical cell is correct? Oxidation occurs

• A. at the anodeCorrect
• B. at the cathode
• C. through the salt bridge
• D. in the aqueous solution

Explanation

In an electrochemical/electrolytic cell, oxidation occurs at the anode, while reduction occurs at the cathode.

Chemistry 2015 Objective — Question 37

Consider the following reaction equation: CuO(s)+H₂(g)→Cu(s)+H₂O(l). Which substance is oxidized?

• A. Cu
• B. CuO
• C. HCorrect
• D. H₂O

Explanation

Oxidation is the addition of oxygen (or loss of electrons); since oxygen is added to H₂ to form H₂O, hydrogen (H) is oxidized.

Chemistry 2015 Objective — Question 38

Which of the following metals is the strongest reducing agent?

• A. Sodium
• B. Silver
• C. PotassiumCorrect
• D. Copper

Explanation

Based on the reactivity/electrochemical series, potassium is the most reactive of the listed metals and therefore the strongest reducing agent.

Chemistry 2015 Objective — Question 39

The complete hydrogenation of benzene gives

• A. cyclohexene
• B. cyclohexaneCorrect
• C. hexene
• D. hexane

Explanation

Complete hydrogenation of benzene (C₆H₆+3H₂) gives cyclohexane (C₆H₁₂), using a nickel catalyst at 180°C.

Chemistry 2015 Objective — Question 40

A compound has an empirical formula CH₂O and molecular mass of 90. Determine its molecular formula. [H=1.00, C=12.0, O=16.0]

• A. C₄H₁₀O₂
• B. C₃H₁₀O₂
• C. C₃H₆O₃Correct
• D. C₇H₂O₄

Explanation

Empirical formula mass = 30. n = 90/30 = 3. Molecular formula = (CH₂O)₃ = C₃H₆O₃.

Chemistry 2015 Objective — Question 41

Which of the following reactions would take place when concentrated sodium hydroxide solution is added to hot palm oil?

• A. Esterification
• B. Neutralization
• C. Polymerization
• D. SaponificationCorrect

Explanation

The reaction of concentrated NaOH with hot palm oil (a fat/oil) produces soap - this process is called saponification.

Chemistry 2015 Objective — Question 42

Starch could be converted to glucose by the process of

• A. condensation
• B. dehydration
• C. fermentation
• D. hydrolysisCorrect

Explanation

Starch is converted to glucose through hydrolysis (breaking it down via addition of water, with the aid of enzymes/acid).

Chemistry 2015 Objective — Question 43

Which of the following compounds is a secondary alkanol?

• A. Ethanol
• B. 2-methylbutan-2-ol
• C. 3-methylpentan-2-olCorrect
• D. Propan-1-ol

Explanation

3-methylpentan-2-ol has its hydroxyl-bearing carbon attached to two alkyl groups, making it a secondary alkanol; ethanol and propan-1-ol are primary, and 2-methylbutan-2-ol (three alkyl groups) is tertiary.

Chemistry 2015 Objective — Question 44

Which of the following substances is a heavy chemical?

• A. Ammonia
• B. Barium hydroxide
• C. Hydrochloric acid
• D. Tetraoxosulphate (VI) acidCorrect

Explanation

A heavy chemical is a substance produced and needed in very large quantities, e.g. tetraoxosulphate (VI) acid (sulphuric acid) and sodium hydroxide, unlike fine chemicals (needed in small/minute amounts).

Chemistry 2015 Objective — Question 45

Which of the following processes do not take place in domestic water treatment?

• A. Chlorination
• B. Flocculation
• C. NeutralizationCorrect
• D. Sedimentation

Explanation

Domestic water treatment involves sedimentation, flocculation/coagulation, filtration and chlorination, but not neutralization.

Chemistry 2015 Objective — Question 46

A substance responsible for the sour taste of unripe oranges is

• A. alkene
• B. alkanol
• C. alkanoic acidCorrect
• D. alkanoate

Explanation

The sour taste of unripe oranges/fruits is due to the presence of an alkanoic acid (e.g. citric/ascorbic acid).

Chemistry 2015 Objective — Question 47

Which of the following products of biotechnology can be used as fuel in place of petrol?

• A. ButaneCorrect
• B. Ethanol
• C. Ethene
• D. Propanol

Explanation

Butane, a gaseous alkane, can be used as a fuel (e.g. in place of petrol/LPG).

Chemistry 2015 Objective — Question 48

Which of the following polymers is thermosetting?

• A. BakeliteCorrect
• B. Nylon
• C. Polypropene
• D. Polystyrene

Explanation

Bakelite is a thermosetting polymer, while nylon, polypropylene and polystyrene are thermoplastics.

Chemistry 2015 Objective — Question 49

The correct balanced equation for the reaction between aluminium metal and hot concentrated tetraoxosulphate (VI) acid is

• A. 2Al(s)+6H₂SO₄(aq)→Al₂(SO₄)₃(aq)+6H₂O(l)+3SO₂(g)Correct
• B. 2Al(s)+3H₂SO₄(aq)→Al₂(SO₄)₃(aq)+6H₂O(l)+3SO₂(g)
• C. 2Al(s)+4H₂SO₄(aq)→Al₂(SO₄)₃(aq)+8H₂O(l)+4SO₂(g)
• D. Al(s)+3H₂SO₄(aq)→Al₂(SO₄)₃(aq)+6H₂O(l)+3SO₂(g)

Explanation

The correctly balanced equation is 2Al+6H₂SO₄→Al₂(SO₄)₃+6H₂O+3SO₂, balancing Al, S, H and O atoms on both sides.

Chemistry 2015 Objective — Question 50

Which of the following gases is monoatomic?

• A. ArgonCorrect
• B. Chlorine
• C. Nitrogen
• D. Oxygen

Explanation

Argon, a noble gas, exists as single (monoatomic) atoms, while chlorine, nitrogen and oxygen exist as diatomic molecules.