WAEC Chemistry 2011 Objective Past Questions

All 50 questions from the West African Examinations Council (WAEC) Chemistry 2011 Objective paper, with the correct answer and a full explanation for each. Free, no signup needed.

Chemistry 2011 Objective — Question 1

How many isotopes has hydrogen?

• A. A. 2
• B. B. 3Correct
• C. C. 4
• D. D. 5

Explanation

Hydrogen has three isotopes: protium, deuterium and tritium.

Chemistry 2011 Objective — Question 2

Which of the following electron configurations correctly represents an inert element?

• A. A. 1s²2s²2p⁴
• B. B. 1s²2s²2p⁶3s²3p⁶
• C. C. 1s²2s²2p⁶3s²3p⁶
• D. D. 1s²2s²2p⁶Correct

Explanation

Whenever a ground state electronic configuration ends in a p⁶ orbital (2p⁶, 3p⁶, 4p⁶...), the element in question is a noble/inert gas. 1s²2s²2p⁶ ends in 2p⁶, matching option D.

Chemistry 2011 Objective — Question 3

What type of reaction is represented by the following equation? ²₁D + ³₁H → ⁴₂He + ¹₀n + energy

• A. A. Nuclear fission
• B. B. Nuclear fusionCorrect
• C. C. Radioactive decay
• D. D. Spontaneous decay

Explanation

This equation exemplifies nuclear fusion. In nuclear fusion, two lighter nuclei combine to give a heavier nucleus; nuclear fission, on the other hand, entails a heavy nucleus splitting into two light nuclei of comparable masses.

Chemistry 2011 Objective — Question 4

Which of the following ions has the electron configuration 2, 8, 8?

• A. A. Na⁺
• B. B. Mg²⁺
• C. C. F⁻
• D. D. Cl⁻Correct

Explanation

11Na = 2,8,1 → Na⁺ = 2,8 (one electron lost). 12Mg = 2,8,2 → Mg²⁺ = 2,8 (two electrons lost). 9F = 2,7 → F⁻ = 2,8 (one electron gained). 17Cl = 2,8,7 → Cl⁻ = 2,8,8 (one electron gained), matching the required configuration.

Chemistry 2011 Objective — Question 5

An element with the electron configuration of 1s²2s²2p⁶ would have a combining power of

• A. A. 0Correct
• B. B. 2
• C. C. 6
• D. D. 8

Explanation

An element whose ground state electronic configuration ends in a p⁶ orbital is a noble gas. Noble gases have a combining power of zero.

Chemistry 2011 Objective — Question 6

Rare gases are stable because they

• A. A. contain equal number of protons and neutrons
• B. B. contain more electrons than protons
• C. C. are chemically active
• D. D. have octet structureCorrect

Explanation

Rare (noble) gases owe their chemical stability to their octet structure (a full outer shell of 8 electrons, or 2 for helium).

Chemistry 2011 Objective — Question 7

Which of the following elements would produce coloured ions in aqueous solution?

• A. A. Calcium
• B. B. IronCorrect
• C. C. Magnesium
• D. D. Sodium

Explanation

Formation of coloured ions is a feature that transition elements are known for. Iron is a transition element.

Chemistry 2011 Objective — Question 8

The energy change that accompanies the addition of an electron to an isolated gaseous atom is

• A. A. bond energy
• B. B. electronegativity
• C. C. electron affinityCorrect
• D. D. ionization energy

Explanation

The energy change that accompanies the addition of an electron to an isolated gaseous atom is known as electron affinity.

Chemistry 2011 Objective — Question 9

Which of the following hydrohalic acids is the weakest?

• A. A. HBr
• B. B. HCl
• C. C. HFCorrect
• D. D. HI

Explanation

The order of increasing acid strength is HF < HCl < HBr < HI, so HF is the weakest hydrohalic acid.

Chemistry 2011 Objective — Question 10

Which of the following arrangements is in order of increasing metallic property?

• A. A. Li<Na<KCorrect
• B. B. Na<Li<K
• C. C. K<Na<Li
• D. D. K<Li<Na

Explanation

Metallic character increases down a group and decreases across a period. Down group 1, Li<Na<K represents increasing metallic property.

Chemistry 2011 Objective — Question 11

Chlorine, bromine and iodine belong to the same group and

• A. A. are gaseous at room temperature
• B. B. form white precipitate with AgNO3(aq)
• C. C. react violently with hydrogen without heating
• D. D. react with alkaliCorrect

Explanation

Chlorine, bromine and iodine are halogens and they all react with alkalis, e.g. chlorine reacts with hot concentrated NaOH to give a solution of sodium chloride and sodium trioxochlorate(V): 3Cl2 + 6NaOH(aq) → 5NaCl(aq) + NaClO3(aq) + 3H2O.

Chemistry 2011 Objective — Question 12

Which of the following elements can conveniently be placed in two groups in the periodic table?

• A. A. Carbon
• B. B. Copper
• C. C. HydrogenCorrect
• D. D. Oxygen

Explanation

Hydrogen can be conveniently placed in both group I and group VII. It fits group I because it has one electron; it fits group VII because it shows non-metallic properties.

Chemistry 2011 Objective — Question 13

The bond formed when two electrons that are shared between two atoms is donated by only one of the atoms is

• A. A. covalent
• B. B. dativeCorrect
• C. C. ionic
• D. D. metallic

Explanation

Dative (coordinate covalent) bonding involves sharing of electrons, but unlike normal covalent bonding, only one of the participating atoms donates both electrons to be shared.

Chemistry 2011 Objective — Question 14

When element ₂₀A combines with element ₈Y,

• A. A. a covalent compound, AY is formed
• B. B. an ionic compound, AY is formedCorrect
• C. C. an ionic compound, A2Y is formed
• D. D. a covalent compound, AY2 is formed

Explanation

20A = 2,8,8,2, so A has a combining power of 2 (loses two electrons). 8Y = 2,6, so Y also has a combining power of 2 (gains two electrons). Using the combining powers, A2+Y2- = A2Y2 = AY. Since electron transfer is involved, the compound formed is ionic.

Chemistry 2011 Objective — Question 15

In metallic solids, the forces of attraction are between the mobile valence electrons and

• A. A. atoms
• B. B. neutrons
• C. C. the negative ions
• D. D. positively charged nucleiCorrect

Explanation

Metallic bonding arises from the attraction between the mobile valence electrons and the positively charged nuclei (metal ions/kernels).

Chemistry 2011 Objective — Question 16

Which of the allowing statements about displacement reaction is correct?

• A. A. A more electropositive element displaces a less electropositive oneCorrect
• B. B. A less electropositive element displaces a more electropositive one
• C. C. The position of elements in the reactivity series has no effect on the reaction
• D. D. It only occurs when the reaction is at equilibrium

Explanation

In a displacement reaction, a more electropositive element displaces a less electropositive element. For instance, zinc can displace copper, but copper cannot displace zinc.

Chemistry 2011 Objective — Question 17

The volume occupied by 17g of H2S at s.t.p. is [H=1.00, S=32.0, Molar volume = 22.4 dm³]

• A. A. 11.2dm³Correct
• B. B. 17.0dm³
• C. C. 34.0dm³
• D. D. 44.8dm³

Explanation

Molar mass of H2S = (1x2)+32 = 34 gmol⁻¹. Mole of H2S = 17g/34gmol⁻¹ = 0.5 mol. Volume = mole x GMV = 0.5 x 22.4 = 11.2 dm³.

Chemistry 2011 Objective — Question 18

Consider the reaction represented by the following equation: xKMnO4(aq)+ySO2(g)+zH2O(l) → K2SO4(aq)+2MnSO4(aq)+2H2SO4(aq); x, y and z are respectively

• A. A. 2, 5 and 2Correct
• B. B. 2, 2 and 5
• C. C. 5, 1 and 2
• D. D. 1, 5 and 2

Explanation

The balanced redox equation between KMnO4 and SO2 is: 2KMnO4(aq)+5SO2(g)+2H2O(l) → K2SO4(aq)+2MnSO4(aq)+2H2SO4(aq). Thus x, y and z are 2, 5 and 2 respectively.

Chemistry 2011 Objective — Question 19

What is the amount of magnesium that would contain 1.20x10^24 particles? [Mg=24, Avogadro's constant=6.02x10^23]

• A. A. 0.5 moles
• B. B. 2.0 molesCorrect
• C. C. 12.0 moles
• D. D. 24.0 moles

Explanation

Mole = Number of particles / Avogadro number = 1.20x10^24 / 6.02x10^23 = 2.0 moles.

Chemistry 2011 Objective — Question 20

The number of atoms in one mole of a substance is equal to the

• A. A. mass number
• B. B. oxidation number
• C. C. atomic number
• D. D. Avogadro numberCorrect

Explanation

The number of particles (atoms, molecules or ions) in one mole of a substance equals the Avogadro number (6.02x10^23).

Chemistry 2011 Objective — Question 21

Which of the following statements about a molar solution is correct? It

• A. A. is a supersaturated solution
• B. B. cannot dissolve more of the solute at that temperature
• C. C. contains a given amount of solute in a given volume of solution
• D. D. contains one mole of the solute in 1 dm³ of solutionCorrect

Explanation

A molar solution contains one mole of the given solute in 1 dm³ of the solution.

Chemistry 2011 Objective — Question 22

A gas that is collected by upward delivery is likely to be

• A. A. heavier than air
• B. B. insoluble in water
• C. C. lighter than airCorrect
• D. D. soluble in water

Explanation

When a gas is collected by upward delivery, it must be less dense (lighter) than air. A gas denser than air is collected by downward delivery.

Chemistry 2011 Objective — Question 23

Bubbling excess carbon (IV) oxide into calcium hydroxide solution results in the formation of

• A. A. CaCO3
• B. B. CaO
• C. C. Ca(HCO3)2Correct
• D. D. H2CO3

Explanation

A small amount of CO2 bubbled into calcium hydroxide forms a white precipitate of CaCO3, but if the carbon (IV) oxide is in excess, Ca(HCO3)2 is formed instead: 2CO2(g)+Ca(OH)2(aq) → Ca(HCO3)2(aq).

Chemistry 2011 Objective — Question 24

The equation P = K/V illustrates

• A. A. Boyle's lawCorrect
• B. B. Charles' law
• C. C. Dalton's law
• D. D. Gay Lussac's law

Explanation

P = K/V illustrates Boyle's law, which states that the volume of a given mass of gas is inversely proportional to pressure at constant temperature: V∝1/P, i.e. V=K/P, which rearranges to P=K/V.

Chemistry 2011 Objective — Question 25

The initial volume of a gas at 300 K was 220cm³. Determine its temperature if the volume became 250cm³.

• A. A. 183 K
• B. B. 264 K
• C. C. 300 K
• D. D. 341 KCorrect

Explanation

Using Charles' law, V1/T1 = V2/T2: 220/300 = 250/T2, so T2 = 300 x 250/220 = 340.91 K ≈ 341 K.

Chemistry 2011 Objective — Question 26

Consider the following energy profile diagram: X represents

• A. A. activated complex
• B. B. activation energyCorrect
• C. C. enthalpy change
• D. D. energy of reactant

Explanation

X represents the activation energy - the minimum amount of energy required for a reaction to occur, shown as the energy 'hump' between reactants and products on the diagram.

Chemistry 2011 Objective — Question 27

Which of the following equimolar solutions has the highest conductivity?

• A. A. CH3COOH(aq)
• B. B. H2CO3(aq)
• C. C. H2SO4(aq)Correct
• D. D. NaOH(aq)

Explanation

Of the given equimolar solutions (CH3COOH, H2CO3, H2SO4 and NaOH), H2SO4 has the highest conductivity, as it is a strong diprotic acid that ionizes fully to release more ions.

Chemistry 2011 Objective — Question 28

The colour of phenolphthalein indicator in alkaline solution at the end-point of an acid-base titration is

• A. A. colourless
• B. B. orange
• C. C. pinkCorrect
• D. D. yellow

Explanation

Phenolphthalein is colourless in acidic medium, pale pink in neutral medium and pink/red in alkaline medium.

Chemistry 2011 Objective — Question 29

Which of the following statements about enthalpy of neutralization is correct? It

• A. A. is constant for a strong acid and a strong baseCorrect
• B. B. cannot be determined using calorimeter
• C. C. has a positive value
• D. D. is higher for a strong acid and a weak base

Explanation

The enthalpy of neutralization for a strong acid and a strong base is constant, with a value of about -57.3 kJmol⁻¹.

Chemistry 2011 Objective — Question 30

When NH4Cl was dissolved in water, the container was cold to touch. This implies that the process is

• A. A. endothermicCorrect
• B. B. the process is exothermic
• C. C. NH4Cl is highly soluble in water
• D. D. NH4Cl forms a saturated solution

Explanation

Since the container was cold to touch, heat was absorbed from the surroundings - i.e. the dissolution is an endothermic process. An exothermic process would make the container feel warm/hot.

Chemistry 2011 Objective — Question 31

Which of the following metallic oxides is amphoteric?

• A. A. Al2O3Correct
• B. B. Fe2O3
• C. C. MgO
• D. D. Na2O

Explanation

Al2O3 is an amphoteric oxide - a metallic oxide that can react with both acids and bases. Another common amphoteric oxide is ZnO.

Chemistry 2011 Objective — Question 32

On evaporation to dryness, 250cm³ of saturated solution of salt X with relative molar mass 101 gave 50.5g of the salt. What is the solubility of the salt?

• A. A. 1.0 moldm⁻³
• B. B. 2.0 moldm⁻³Correct
• C. C. 4.0 moldm⁻³
• D. D. 5.0 moldm⁻³

Explanation

Mole = mass/molar mass = 50.5/101 = 0.5 mol. Volume = 250cm³ = 0.25 dm³. Solubility = mole/volume = 0.5/0.25 = 2 moldm⁻³.

Chemistry 2011 Objective — Question 33

Consider the following reaction equation: X(g)+Y(g)⇌XY(g); ΔH=+220kJmol⁻¹. If the temperature of the system is increased, the

• A. A. backward reaction would be favoured
• B. B. forward reaction would be favouredCorrect
• C. C. reaction would stop
• D. D. reaction would be at equilibrium

Explanation

The positive ΔH shows the forward reaction is endothermic. An increase in temperature favours the endothermic (forward) reaction, while a decrease favours the exothermic (backward) reaction.

Chemistry 2011 Objective — Question 34

Which of the following conditions would lead to an increase in the rate of a reaction?

• A. A. Increase in temperature and decrease in the surface area of reactants
• B. B. Increase in temperature and concentration of reactantsCorrect
• C. C. Decrease in temperature and increase in concentration of reactants
• D. D. Decrease in temperature and increase in the surface area of reactants

Explanation

As temperature and concentration both increase, the rate of a reaction increases; both factors provide more energetic, more frequent collisions between reacting particles.

Chemistry 2011 Objective — Question 35

What is the value of n in the following equation? CrO4²⁻+14H⁺+ne⁻→2Cr³⁺+7H2O

• A. A. 2
• B. B. 3
• C. C. 6Correct
• D. D. 7

Explanation

Equating overall charge on both sides: L.H.S. = -2+14-n = 12-n. R.H.S. = (2x+3)+(7x0) = +6. Equating: 12-n = 6, so n = 6.

Chemistry 2011 Objective — Question 36

What mass of copper would be formed when a current of 10.0A is passed through a solution of CuSO4 for 1 hour? [Cu=63.5; 1F=96500C]

• A. A. 5.9 g
• B. B. 11.8 gCorrect
• C. C. 23.8 g
• D. D. 47.3 g

Explanation

Q = It = 10 x 3600 = 36000 C. Cu²⁺+2e⁻→Cu(s) needs 2F (193000C) to deposit 1 mole (63.5g) of copper. Mass deposited by 36000C = (63.5/193000) x 36000 = 11.8 g.

Chemistry 2011 Objective — Question 37

Which of the following metals could be used as sacrificial anode for preventing the corrosion of iron?

• A. A. Copper
• B. B. Lead
• C. C. MagnesiumCorrect
• D. D. Silver

Explanation

A sacrificial anode metal must be higher than the metal it protects in the electrochemical series. Since magnesium is higher than iron in the series, it can serve as a sacrificial anode to protect iron from corrosion.

Chemistry 2011 Objective — Question 38

Consider the following electrochemical cell notation: M(s)/M²⁺(aq)//H⁺(aq)/H2(g). The value of the electrode potential is positive when

• A. A. electrons flow from the metal electrode, M(s) to hydrogen electrode, H2(g)Correct
• B. B. electrons flow from hydrogen electrode, H2(g) to metal electrode, M(s)
• C. C. the flow of current is high
• D. D. there is equilibrium between the flow of electrons from the hydrogen electrode to metal electrode

Explanation

The value of the electrode potential is positive when electrons flow from the metal electrode to the hydrogen electrode.

Chemistry 2011 Objective — Question 39

Which of the following compounds determines the octane rating of petrol?

• A. A. 1,2,3-trimethylpentane
• B. B. 2,3,5-trimethyloctane
• C. C. 2,3,5-trimethylpentane
• D. D. 2,2,4-trimethylpentaneCorrect

Explanation

2,2,4-trimethylpentane (isooctane) determines the octane number of a petrol sample. Octane number = (Mass of 2,2,4-trimethylpentane / Mass of mixture) x 100.

Chemistry 2011 Objective — Question 40

Which of the following compounds would react with ethanoic acid to give a sweet-smelling liquid?

• A. A. Alkane
• B. B. AlkanolCorrect
• C. C. Alkanal
• D. D. Alkyne

Explanation

Alkanols react with alkanoic acids to produce alkanoates, which are sweet-smelling compounds. For instance, ethanol reacts with ethanoic acid to give ethyl ethanoate.

Chemistry 2011 Objective — Question 41

Which of the following separation techniques would show that black ink is a mixture of chemical compounds?

• A. A. Crystallization
• B. B. ChromatographyCorrect
• C. C. Filtration
• D. D. Sublimation

Explanation

Black ink can be shown to be a mixture of different chemical compounds by chromatography, which separates coloured mixtures into their component substances.

Chemistry 2011 Objective — Question 42

The following substances are examples of addition polymer except

• A. A. nylonCorrect
• B. B. Perspex
• C. C. polyethane
• D. D. polychloroethane

Explanation

Nylon is a condensation polymer (e.g. nylon-6,6 from condensation polymerization of hexane-1,6-diamine and hexanedioic acid), unlike Perspex, polyethene and polychloroethene, which are addition polymers.

Chemistry 2011 Objective — Question 43

When bromine is added to ethene at room temperature, the compound formed is

• A. A. 1,1-dibromoethane
• B. B. 1,1-dibromoethene
• C. C. 1,2-dibromoethaneCorrect
• D. D. 1,2-dibromoethene

Explanation

Bromine reacts at room temperature with ethene to give 1,2-dibromoethane: Br2+CH2CH2→CH2BrCH2Br.

Chemistry 2011 Objective — Question 44

Which of the following organic compounds would react with sodium trioxocarbonate (IV) to liberate carbon (IV) oxide?

• A. A. (structure of ethanoic acid: H-C(H2)-COOH)Correct
• B. B. (ester structure)
• C. C. (aldehyde structure: HC(=O)CH2CH3)
• D. D. (ketone structure: CH3C(=O)CH3)

Explanation

Acids react with carbonates and hydrogen carbonates to liberate carbon (IV) oxide. Structure A is ethanoic acid (a carboxylic acid, -COOH group), so it will react with Na2CO3 to liberate CO2.

Chemistry 2011 Objective — Question 45

The compound that makes palm wine taste sour after exposure to the air for few days is

• A. A. ethanol
• B. B. ethanoic acidCorrect
• C. C. methanol
• D. D. methanoic acid

Explanation

Ethanoic acid is produced when bacteria in the wine convert the ethanol present into ethanoic acid, which has a sour taste.

Chemistry 2011 Objective — Question 46

The reagent that can be used to distinguish ethene from ethyne is

• A. A. ammoniacal silver trioxonitrate (V) solutionCorrect
• B. B. Benedict solution
• C. C. bromine water
• D. D. Fehling's solution

Explanation

Ammoniacal AgNO3, ammoniacal CuCl and sodium in liquid NH3 can distinguish terminal alkynes (like ethyne) from other organic compounds such as ethene, since terminal alkynes have a triple bond at the extreme end.

Chemistry 2011 Objective — Question 47

The following substances are ores of metals except

• A. A. bauxite
• B. B. cuprite
• C. C. cassiterite
• D. D. graphiteCorrect

Explanation

Bauxite is an ore of aluminium, cuprite is an ore of copper, cassiterite is an ore of tin; graphite is not an ore at all - it is one of the crystalline allotropes of carbon (the other being diamond).

Chemistry 2011 Objective — Question 48

Which of the following processes does not involve the use of limestone?

• A. A. Extraction of iron in the blast furnace
• B. B. Manufacture of tetraoxosulphate (VI) acid by Contact processCorrect
• C. C. Production of washing soda by Solvay process
• D. D. Production of cement

Explanation

The manufacture of H2SO4 by the Contact process does not require limestone at all; it requires sulphur, oxygen, pure H2SO4, water and V2O5 as catalyst.

Chemistry 2011 Objective — Question 49

Which of the following substances is mainly responsible for the depletion of the ozone layer?

• A. A. ChlorofluorocarbonCorrect
• B. B. Carbon (IV) oxide
• C. C. Nitrogen
• D. D. Oxygen

Explanation

Chlorofluorocarbons (CFCs) are the main substances responsible for the depletion of the ozone layer, which protects the earth from receiving too much UV radiation.

Chemistry 2011 Objective — Question 50

Aluminium is extracted electrolysis from

• A. A. bauxiteCorrect
• B. B. cryolite
• C. C. duralumin
• D. D. kaolin

Explanation

Aluminium is extracted by the electrolysis of bauxite (cryolite is used to lower the melting point of the electrolyte, not as the source of aluminium).