WAEC Chemistry 2025 Theory Past Questions

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Molar mass of O3 = 16 x 3 = 48 g/mol. Number of moles = mass/molar mass = 4.8/48 = 0.1 mole.

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HSO4-

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NO-

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Buta-2-ene (2-butene)

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Propanoic acid

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It means the neutral atom has no electrical charge; the number of protons is equal to the number of electrons in the atom.

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1s2 2s2 2p6 3s2

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1s2 2s2 2p6 (Na+ has 10 electrons; it loses 1 electron from the neutral Na atom, 1s2 2s2 2p6 3s1)

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1s2 2s2 2p6 (N3- has gained 3 electrons to have a total of 10 electrons)

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An acid salt is one in which the replaceable hydrogen atoms are partially replaced by a metal or ammonium ion; a salt which still contains replaceable hydrogen ions, e.g. KHSO4, NaHSO4. They are formed from a strong acid and a weak base and produce an acidic solution when dissolved in water, e.g. NH4Cl(aq) -> NH4+(aq) + Cl-(aq).

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a = mass number/atomic mass; b = atomic number/proton number.

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Filtration

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Magnetic separation

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Fractional distillation

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Metallic bond

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Ionic bond

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Covalent bond. In PH3 (phosphine), the central phosphorus atom shares three of its five valence electrons with three hydrogen atoms, each contributing one valence electron, forming three P-H covalent bonds. The remaining two valence electrons form a lone pair, giving PH3 a polar, trigonal pyramidal shape (H-P-H bond angle ~93.5 degrees, P-H bond length ~1.42 Angstrom).

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Ag < Pb < Zn < Mg < Na

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Na (sodium) donates electrons most easily; it has one valence electron far from the nucleus.

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A functional group is an atom, group of atoms, or bond which determines the chemical properties of a compound.

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A primary alkanol is an alkanol with the hydroxyl group (-OH) attached to a carbon atom that is bonded to only one other alkyl group (or none). It can be represented as R-CH2-OH, e.g. methanol (CH3OH), ethanol (CH3CH2OH).

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Hexanol: C6H14O / -OH (hydroxyl). Propyl butanoate: C7H14O2 / -COO- (ester). Ethene: C2H4 / -C=C- (alkene).

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I. Polymerization. II. Combustion. III. Oxidation/addition reaction.

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Used for making plastics, electrical insulators, packaging material, transparent films, and garbage bags.

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Carbon (II) oxide (carbon monoxide, CO)

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Hydrochloric acid (HCl) and Tetraoxosulphate (VI) acid (H2SO4); trioxonitrate (V) acid (HNO3) is also acceptable.

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Using CaVa/CbVb = na/nb, where na = 1 (KOH), nb = 2 (H2SO4), Ca = 0.1 mol/dm3, Va = 12.50 cm3, Cb = 0.1 mol/dm3: Vb = (Ca x Va x nb)/(na x Cb) = (0.1 x 12.50 x 2)/(1 x 0.1) = 25.0 cm3.

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Electrolysis is the use of electric current (direct current, DC) to chemically decompose a compound into its constituent elements or ions.

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Brine is a concentrated aqueous solution of sodium chloride (NaCl). The anions competing for discharge at the anode are Cl- and OH-. OH- is lower in the electrochemical series (more easily discharged based on position alone), but Cl- is preferentially discharged instead because of its much higher concentration in brine.

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Na < Ca < O < Cl

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Na: used in sodium vapour lamps, as coolant in nuclear reactors, as a reducing agent, in production of NaOH, Na2O2 and NaCN. Cl: used in manufacture of HCl, as a bleaching agent, as a germicide in water treatment, in manufacture of plastics, dyes and drugs. Ca: used in deoxidation in steel casting/copper alloys, formation of healthy teeth and bones. O: used as an antiseptic/disinfectant, as a raw material in the production of steel, in the oxy-ethyne flame for welding, as an oxidizer in rocket fuel, and in respiration/life support/sea diving.

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X: Water (H2O). Y: Calcium hydroxide, Ca(OH)2. Z: Ammonia, NH3.

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I. CaO + H2O -> Ca(OH)2. II. Ca(OH)2 + 2NH4Cl -> CaCl2 + 2NH3 + 2H2O.

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4Al + 3O2 -> 2Al2O3

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From 4Al + 3O2 -> 2Al2O3: 4(27)g Al reacts with 3(32)g O2, i.e. 108g Al reacts with 96g O2. For 9g Al: x = (9 x 96)/108 = 8.0g of O2.

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A disproportionation reaction is a redox reaction in which a substance is simultaneously oxidized and reduced (acts as both oxidizing and reducing agent), giving products with different oxidation states, e.g. decomposition of hydrogen peroxide (2H2O2 -> 2H2O + O2), and the reaction of sodium hydroxide with chlorine gas (NaOH + Cl2 -> NaCl + NaClO + H2O).

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I. POCl3: P = +5. II. PH3: P = -3. III. P4: P = 0. IV. H3PO3: P = +4.

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Sulphur (added in the vulcanization process, which forms cross-links between the rubber's molecules for improved strength, toughness and durability).

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Dalton's law of partial pressure states that the total pressure exerted by a mixture of gases that do not react together chemically is the sum of the partial pressures of the individual gases: PT = PA + PB + PC + ..., where PA, PB, PC are the partial pressures of the individual gases.

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Total moles = 2+3+5 = 10 moles. Mole fraction of O2 = 2/10; Partial pressure of O2 = (2/10) x 210 = 42.0 Nm-2. Mole fraction of N2 = 5/10; Partial pressure of N2 = (5/10) x 210 = 105 Nm-2.

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Concentration of a solution is defined as the amount (in moles) of solute present in 1 dm3 of the solution.

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Mass concentration = molar concentration x molar mass = 0.10 x 106 = 10.6 g dm-3.

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Weigh 10.6g of Na2CO3. Dissolve it in distilled water in a beaker. Transfer the content of the beaker into a 1 dm3 volumetric flask. Add distilled water up to the graduation mark.

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Chlorine: greenish-yellow gas, pungent/irritating smell, moderately soluble in water, denser than air. Hydrogen chloride: colourless gas, sharp/irritating smell, denser than air, highly soluble in water.

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The greenish-yellow colour of chlorine fades / misty fumes of hydrogen chloride are formed.

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HCl is used to perform the fountain experiment. CH4 is used as a domestic fuel.

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HCl is used for the fountain experiment because it is highly soluble in water. CH4 is used as a domestic fuel because it has a high calorific value.

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Used in the synthesis of chloroethane; used in removing rust/scales from metals (pickling); used in the tanning industry to convert animal hides/skins into leather.

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Producer gas

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Carbon (II) oxide (CO) and Nitrogen (N2)

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This is because they have the same/similar molecular weight (CO = 12+16 = 28 g/mol; N2 = 14x2 = 28 g/mol).

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Nitrogen (N2)

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This is because a high amount of energy is required to break the triple bond in the nitrogen molecule.

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2Na(s) + 2H2O(l) -> 2NaOH(aq) + H2(g)

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Fe2O3 + 3CO -> 2Fe + 3CO2, and 2Fe2O3 + 3C -> 4Fe + 3CO2.

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This is because the reaction that occurs in the lower part of the furnace is exothermic (releasing heat), while the reaction that occurs in the upper part is endothermic (absorbing heat).

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Ammoniacal liquor: manufacture of fertilizer. Coal tar: manufacture of dyes, drugs and explosives, and road construction. Coke: production of water gas. Coal gas: used as fuel.

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Al(OH)3: extraction/purification of bauxite, dyeing industry. KAl(SO4)2 (alum): water treatment. Na3AlF6 (cryolite): aluminium production/extraction of aluminium.

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Al(OH)3: seeding of crystals. KAl(SO4)2: coagulation of solid particles, helps the dye stick to the cloth. Na3AlF6: dissolves alumina and lowers its melting point for electrolysis.

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Potassium aluminium tetraoxosulphate (VI) [potash alum]

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The equation of reaction is: NaHCO3(aq) + HCl(aq) -> NaCl(aq) + CO2(g) + H2O(l). A is put in the burette and titrated against 20.0 cm3 or 25 cm3 portions of B using methyl orange as indicator; average titre (volume of A used) = 18.30 cm3, from rough and 1st/2nd titre readings of 18.20 and 18.40 cm3.

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250 cm3 of HCl contains 0.825g (0.82g rounded). 1000 cm3 will contain (1000 x 0.825)/250 = 3.30 g/dm3. Molar mass of HCl = 1 + 35.5 = 36.5 g/mol. Concentration of A in mol/dm3 = 3.30/36.5 = 0.0904 mol dm-3.

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From the equation, mole ratio NaHCO3 : HCl = 1:1, so nA:nB = 1:1. Using CaVa/CbVb = na/nb: Ca = 0.0904 mol/dm3, Va = 18.30 cm3, Vb = 25.0 cm3, na = nb = 1. Cb = (Ca x Va)/(Vb) = (0.0904 x 18.30)/25 = 0.067 mol dm-3.

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Molar mass of NaHCO3 = 23+1+12+(16x3) = 84 g/mol. Concentration in g/dm3 = conc. in mol/dm3 x molar mass = 0.067 x 84 = 5.59 g/dm3.

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% purity = (mass concentration of pure NaHCO3 / mass concentration of impure NaHCO3) x 100%. Impure B: 250cm3 contains 4.0g, so 1000cm3 contains (1000x4)/250 = 16.0 g/dm3. % purity = (5.59/16.0) x 100% = 34.94%.

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(a) C + distilled water + litmus: dissolves to give a blue solution, turns blue litmus red -> transition metal salt/Cu2+ present, solution is acidic. (b)(i) First portion + NaOH drops then excess: blue precipitate forms, dissolves in excess -> Cu2+ present. (ii) Second portion + NH3 drops then excess: light/pale blue precipitate forms, dissolves in excess to give a deep blue solution -> Cu2+ present. (iii) Third portion + BaCl2 then dilute HCl: white precipitate forms, insoluble in HCl -> SO4^2- present. (iv) Fourth portion + Fehling's solution, boil: brick red/orange precipitate forms -> reducing sugar/glucose present.

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R is ammonia gas, NH3

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Hold a stopper of concentrated hydrochloric acid near the gas: dense white fumes of ammonium chloride are formed.

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(NH4)2SO4

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Ammonium tetraoxosulphate (VI)