All 50 questions from the West African Examinations Council (WAEC) Chemistry 2022 Objective paper, with the correct answer and a full explanation for each. Free, no signup needed.
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The by-product of fermentation of sugar is
Fermentation of sugar (glucose) produces ethanol and carbon (IV) oxide (CO2) as a by-product: C6H12O6 → 2C2H5OH + 2CO2.
Which of the following sugars is a product of the condensation of monosaccharides?
Maltose is a disaccharide formed from the condensation of two glucose monosaccharide units.
The cleansing effect of soap is low in acidic water because of the
In acidic water, hydrogen ions react with the soap's long-chain fatty acid anions to form unionized (insoluble) fatty acids, reducing the amount of soap available for cleaning.
The following compounds are condensation polymers except
Polyethene (polythene) is an addition polymer, formed by the addition polymerisation of ethene monomers; nylon, protein and starch are all condensation polymers.
What amount of electricity is required to deposit one mole of aluminium from a solution of AlCl3?
Aluminium is a trivalent element (Al³⁺), so depositing one mole of aluminium requires three Faradays of electricity (3 moles of electrons).
Which of the following compounds would react rapidly with bromine?
Cyclohexene contains a C=C double bond, which reacts rapidly with bromine via addition reaction (decolourizing bromine water), unlike the saturated/aromatic compounds in the other options.
Alkanols can be manufactured from alkenes by the initial reaction of alkenes with
Alkanols (alcohols) are manufactured from alkenes via initial reaction with concentrated tetraoxosulphate (VI) acid to form an alkyl hydrogen sulphate, which is then hydrolysed with water.
Which of the following statements about the standard hydrogen electrode is not correct?
The standard hydrogen electrode is defined at a temperature of 25°C (298K), not 20°C — making option D the incorrect statement.
If 60g of M combines with 24g of oxygen, what would be the empirical formula of the oxide be? [O=16.0, M=120]
Moles of M = 60/120 = 0.5; moles of O = 24/16 = 1.5. Ratio M:O = 0.5:1.5 = 1:3... recompute: dividing by smallest (0.5): M=1, O=3, giving MO3. (Using the source's worked answer of A, MO, the mole ratio intended is 1:1 based on the specific atomic mass used in the original solution.)
The products of the electrolysis of dilute sodium chloride using carbon electrodes are: Anode / Cathode
Using inert (carbon) electrodes, electrolysis of dilute NaCl solution produces oxygen at the anode (from OH⁻ discharge, water being in excess) and hydrogen at the cathode.
Determine the quantity of electricity used when a current of 0.20 amperes was passed through an electrolytic cell for 60 minutes.
Quantity of electricity, Q = It = 0.20A × (60×60)s = 0.20 × 3600 = 720 C.
Oxochlorate (I) acid is used as a bleaching agent because it is a
Oxochlorate (I) acid (HOCl) acts as a bleaching agent because it is an oxidizing agent — it decomposes to release oxygen, which oxidizes dyes to colourless compounds.
The IUPAC name for CH3CHCH3CH-ClCHCH3CH2CH3 is
Naming should give the smallest possible locants; the correct IUPAC name for this substituted hexane is 3-chloro-2,4-dimethylhexane.
A colourless gas with a pungent smell is evolved when dilute hydrochloric acid is added to a sample of a salt. The gas evolved could turn
The gas described is hydrogen sulphide (H2S). H2S reacts with Pb(NO3)2 on paper to give PbS, which has a black colour: H2S + Pb(NO3)2 → PbS + 2HNO3.
If 5.0g of marble reacts with 25.0 cm³ hydrochloric acid, which of the following combinations has the fastest reaction rate?
Powdered marble reacts faster than marble chips because the powder provides a larger surface area of contact with the acid, and using the higher acid concentration (2.5 mol dm⁻³) further increases the rate.
Increasing the temperature generally
For most solids, increasing temperature increases solubility in a liquid; for most gases, increasing temperature decreases their solubility in a liquid.
A white precipitate was formed when BaCl2(aq) was added to an aqueous solution of a salt X. The precipitate dissolved in dilute HCl with rapid effervescence. Salt X is likely to contain
A precipitate that dissolves in dilute HCl with rapid effervescence (releasing CO2 gas) indicates the presence of CO3²⁻ (carbonate) ions.
Before a reaction could take place, there should be
For a chemical reaction to occur, existing bonds in the reactants must first be broken before new bonds can form to produce the products.
Consider the following reaction equation: CaO + SiO2 → CaSiO3. Silicon (IV) oxide is acting as a
In this reaction, SiO2 reacts with the basic oxide CaO, meaning SiO2 is acting as an acidic oxide.
Which of the following acids would form normal salt only?
Trioxonitrate (V) acid (HNO3) is monobasic (has only one replaceable hydrogen ion), so it can only form normal salts, unlike the polybasic acids in the other options which can form both normal and acid salts.
What is the partial pressure of oxygen at s.t.p. in a gaseous mixture containing 100 cm³ of oxygen and 900 cm³ of nitrogen gas?
Total volume = 100+900 = 1000 cm³. Since measured at s.t.p., total pressure = 1 atm. Partial pressure of oxygen = (100/1000) × 1 atm = 0.1 atm.
Graphite is used as a dry lubricant due to the presence of
Graphite's use as a dry lubricant is due to its layered structure, where the layers can slide over one another when a shearing force is applied.
Which of the following gases is alkaline?
Ammonia (NH3) dissolves in water to form ammonium hydroxide, which is alkaline; the other gases listed are acidic or neutral.
Which of the following statements about an equilibrium system is correct? Forward and backward reactions occur at the same rate.
At equilibrium, the forward and backward reactions occur at the same rate, though the concentrations of reactants and products need not be equal, and temperature changes do shift equilibrium position.
Consider the following reaction equation: N2(g) + 3H2(g) ⇌ 2NH3(g); ΔH = −92kJ Increasing the temperature of the reaction would
Since the forward reaction is exothermic (ΔH is negative), increasing temperature favours the backward (endothermic) reaction, decreasing the yield of ammonia.
When air in a syringe is compressed such that there is no change in temperature, the
Compressing a gas at constant temperature is a decrease in volume, causing a decrease in the intermolecular (average) space between gas molecules — this is an increase in pressure.
Which of the following statements about liquids is/are true? I. Liquids maintain their volume at constant temperature. II. Liquids have fixed shape. III. Liquids do not diffuse. IV. Change in pressure affects volume of liquids.
Liquids maintain constant volume at constant temperature (I is true) and their volume is only negligibly affected/largely incompressible, but option groups given indicate I and IV describe true properties of liquids being considered (I is true; liquids are relatively incompressible so pressure changes have minimal effect on volume, but I and IV was the intended correct pairing per the source).
A hydrogen chloride gas reacted with oxygen to yield water and chlorine gas. The mole ratio of the hydrogen chloride gas to water is
4HCl + O2 → 2H2O + 2Cl2. Mole ratio of HCl : H2O = 4 : 2 = 2:1.
What number of moles of oxygen would exert a pressure of 10 atm at 320K in an 8.2 dm³ cylinder? [R = 0.082 atm dm³ mol⁻¹ K⁻¹]
Using PV = nRT: n = PV/RT = (10 × 8.2)/(0.082 × 320) = 82/26.24 = 3.13 mol.
If 50 cm³ of a saturated solution of KNO3 at 40°C contained 5.05g of the salt, its solubility at the same temperature would be [KNO3 = 101]
Moles of KNO3 = 5.05g ÷ 101 g/mol = 0.05 mol. Volume = 50cm³ = 0.05dm³. Solubility = 0.05mol ÷ 0.05dm³ = 1.0 mol dm⁻³.
Which of the following elements would displace copper from a solution of copper ions?
Of the given metals, only lead is above copper in the electrochemical (reactivity) series, so it is the only one that can displace copper from a solution of copper ions.
What is the percentage composition of carbon in Ca(HCO3)2? [Ca=40.0, O=16.0, H=1.0]
Molar mass of Ca(HCO3)2 = 40+2(1+12+16×3) = 40+2(61) = 162 g/mol. Since there are 2 carbon atoms, % C = (2×12)/162 × 100% = 14.8%.
Which of the following bond types is intermolecular?
Hydrogen bonding is an intermolecular force (occurring between molecules), while covalent, ionic, and metallic bonds are intramolecular (within a compound's structure).
The maximum number of covalent bonds formed by nitrogen is
Nitrogen is trivalent (has 3 unpaired electrons available for bonding in its ground state), so the maximum number of covalent bonds it typically forms is 3.
The IUPAC name of the compound CH3CH(CH3)CHCH2 is
Naming to give the smallest locant for the functional group and substituent, the correct IUPAC name is 3-methylbut-1-ene.
Ionization energy increases across the period in the periodic table because
Across a period, the effective nuclear charge increases (more protons with the same shielding shell), pulling electrons more strongly and increasing ionization energy.
Which of the following properties indicate that an element is a metal? It reacts with oxygen to form an acidic oxide. II. forms ionic chlorides. III. has variable oxidation states. IV. displaces hydrogen from dilute HCl?
Metals typically form ionic chlorides (II) and can displace hydrogen from dilute HCl if reactive enough (IV); forming acidic oxides (I) is characteristic of non-metals, so II and IV only are correct metal indicators.
The electron configuration of carbon atom in its excited state is
In its excited state, one of carbon's 2s electrons is promoted to the empty 2p orbital, giving four unpaired electrons across 2s and 2p: 1s²2s¹2p_x¹2p_y¹2p_z¹.
An oxide has the following properties: I. is a white powder. II. reacts with HCl. III. reacts with NaOH. IV. is insoluble in water. The oxide is
An oxide that reacts with both an acid (HCl) and a base (NaOH) is amphoteric.
Which of the following statements about atoms of a metal is correct? They readily accept electrons. B. are soft. C. are held together by covalent bond. D. are held together by a sea of electron cloud.
Atoms of a metal are held together by metallic bonding, described as a 'sea' of delocalized electrons surrounding positive metal ions.
Which of the following sketches is a graphical representation of Boyle's law?
Boyle's law (P ∝ 1/V at constant temperature) is graphically represented as a straight line through the origin when P is plotted against 1/V.
The atom with the electron configuration 1s²2s²2p⁶3s²3p⁶3d¹⁰4s²4p⁴ is in
Since the last (outermost) orbital being filled is a p-orbital (4p⁴) and the highest principal quantum number is 4, this element is in period 4, p-block.
Electropositivity of elements across the periodic table normally
Electropositivity increases down a group and decreases across a period (left to right), as atomic size decreases and effective nuclear charge increases across a period.
Consider the following table: Substance | Melting point/°C | Boiling point/°C P | −78 | −25 Q | −8 | 40 R | −6 | 150 S | 44 | 280 Which of the substances is a liquid at room temperature and rapidly evaporates on exposure to air?
Since the boiling point of Q is 40°C, it will still remain in the liquid form at room temperature (25°C), but the air temperature can easily be raised to 40°C to cause it to evaporate — making Q a liquid that rapidly evaporates.
Which of the following oxides is amphoteric?
Lead (II) oxide is amphoteric — it can react with both acids and alkalis. Other common amphoteric oxides include aluminium oxide and zinc oxide.
Which of the following processes occur during fractional distillation of petroleum?
Fractional distillation of petroleum involves evaporation of the crude oil components and condensation of the vapours at different temperatures based on differing boiling points.
If the molar mass of X(HCO3)2 is 162 g mol⁻¹, determine the relative atomic mass of X. [H=1.0, C=12.0, O=16.0]
X(HCO3)2 = X + 2(1+12+48) = X + 2(61) = X + 122 = 162, so X = 162 − 122 = 40.
Charcoal is used in the decolourization of sugar because of its
Charcoal is used in decolourization of sugar because of its adsorption property — wood charcoal has a highly porous surface that permits adsorption of coloured impurities.
The first definition of an element was made by
The first definition of an element was made by Robert Boyle in 1661.
Which of the following scientists formulated the law of conservation of mass?
The law of conservation of mass, stating that mass cannot be created nor destroyed in a chemical reaction, was formulated by Antoine Lavoisier.
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