Chemistry 2020 Past Questions | WAEC

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Study the following Chemistry past questions and answers for JAMB, WAEC NECO and Post-JAMB. Get prepared with official past questions and answers for upcoming examinations.

46. which of the following quantities is a molar quantity?

A. Molarity

B. Molar mass
C. Mass concentration
D. Molality

47. Dilution factor is the?

A. amount of distilled water that is added to the concentrated solution to dilute it
B. Quantity of distilled water added to l dm33 of the concentrated solution to give the dilute solution

C. number of times the volume of the concentrated solution is diluted to give the dilute solution
D. act of diluting the concentrated solution to obtain the dilute solution

48. Calcium chloride is an ionic compound. Which of the following statements account for its ionic character?

Calcium has high ionization energy. II. Calcium has low ionization energy. III. Chlorine has high electron afinity. IV. Chlorine has high 1onization energy.

A. I and Il only

B. 1, II and IV only
C. II, IIl and IV only
D. I, II, III and IV

49. An example of a biodegradable pollutant is?

A. plastic
B. sewage

C. carbon (II) oxide
D. hydrogen sulphide

CHEMISTRY 2020 WAEC QUESTIONS THEORY WITH SOLUTIONS

(a)(i) State Faraday’s first law of electrolysis. (ii) Distinguish between a strong electrolyte and a weak electrolyte

(b) State one chemical property of ethyne.

(c)( i) What is meant by the tern unsaturated hydrocarbon? (ii) Complete the following reaction equation: CH₃ + CH₃OH-> (iii) Name the major product formed in the cation stated in 1(c)(ii).

(d) State one way by which the rate of esterification could be increased.

(e) Consider the reaction represented by the following equation: Zn + H₂SO₄ → ZnOS₄ + H₂ . If 3.75g of Zn dust was added to excess H₂SO₄. Calculate the number of molecules of hydrogen gas produced. [ Zn = 65.0, Na = 6.02 X10₂₃ ].

(f) State one effect of global warming.

(g) Consider the following reaction equation:

Pb(NO₃) +H₂S –> PbS + 2HNO₃;

H₂+ C₂H₄→ C₂H₆.
Zn(OH)₂+ 2OH → [ Zn(OH)₄]₂.

(i) Which of the equations represent(s) redox process? (ii) State the change in Oxidation number of the species that are oxidized or reduced. (h)(i) State two of the main concepts of Bohr’s model of the atom. (ii) State the limitations of Bohr’s model. (i) List three factors that could influence the equilibrium position of a reversible reaction. (j) Calcium trioxocarbonate(iv) powder is added to separate equimolar solutions of hydrochloric acid and ethanoic acid. State one: (i) similarity in the observation in both reactions: (ii) difference in the observation in both reactions.

(a) Consider the following compounds: (i) What is the relationship between the compounds labeled A and B? (ii) Name each of compounds A and B. (iii) Will the chemical properties of compounds A and B be the same? (iv) Give the reason for the answer stated in 2(a)(ii).

(b)(i) Give two characteristic features of boiling, (ii) What would be the effect of each of the following conditions on the boiling point of water? I. Addition of crystals of sodium chloride. II. Reduction of the atmospheric pressure. (iii) State one way in which boiling differs from evaporation.

(c)(i) Differentiate between an unsaturated solution and a saturated Solution. (ii) State two ways by which a saturated solution could be made to dissolve more solute. (iii) State one factor that could affect the solubility of a solid in a liquid. (d)(i) Define the term mole .

(ii) Consider the following reaction equation: MgO + 2HCI → MgCl₂ + H₂. What mass of magnesium Oxide is needed to neutralize 25.0 cm3 of 0.1 mol dm-3 hydrochloric acid? [O= 16.0; Mg = 24.0]. (e) State three physical properties of metals.

Solution & Explanation:

(a) (i) Faraday’s first law of electrolysis states that the amount of a substance discharged at/dissolved from an electrode is directly proportional to the quantity of electricity that passes through the electrolyte.

– The mass of an element deposited at the electrode is directly proportional to the quantity of electricity passed.

(ii) Strong electrolytes dissociate/ionize completely in aqueous solutions while weak electrolytes dissociate/ionize only slightly/partially in aqueous solution.

(b)

-it burns in air/oxygen;

– it undergoes addition reaction;-it undergoes polymerization,

– it undergoes substitution/precipitation reaction.

(c)i) hydrocarbon with double/triple bond/multiple bonds between at least a pair of carbon atoms.

(ii) CH₃COOH + CH₃OH ⇌ CH₃COOCH₃ + H₂

(ii) Methylethanoate (d) addition of concentrated H₂SO₄ ; – removal of the ester as soon as it forms; – increase the concentration of one of the reactants.

(e) 65.0g of Zn2+2+ will produce 6.02 x 10²³ molecules of H2, OR 65.0g Zn = I molecule of H2

∴ 3.75g of Zn = 1/3.7565 X 6.02 x 10²³

= 3.4 X 10 molecules

(f) Sea level rising: -severe climate changes,depletion of coral reefs; -alters ecosystem balance/setting animal on the move; – severe drought; flooding or( areas); – polar ice melting

(g)(i) equation B, H₂ +C₂H4 –> C₂H₆

(ii) In equation B, H changes from zero to-1; C changes from +2 to +3 as Concentration (i)

(h)(i) an electron in an atom exists/revolves a circular orbit; – energy of an electron is quantified / has a fixed value; – an electron emits energy in the form of radiation when it moves from a higher energy state to a lower energy State.

(ii) Bohr’s model cannot explain the more complicated spectra lines observed in spectra Other than that of hydrogen. (i) temperature; pressure/volume; -concentration

(j) (i) produces a gas/effervescence/bubbles- given off;- HCI will produce a more vigorous reaction/CH₃COOH will produce a less vigorous reaction.

(a) In the laboratory preparation of dry chlorine gas, state the: I. reagents used; II. drying agent III. the mode of collection.

(i) Write the equation for the preparation of chlorine gas. (iii) Write an equation to show how chlorine reacts with hot concentrated NaOH.

(b)i). Name the main raw materials used for the extraction of iron in the blast furnace. (ii) Write the equations of the reactions taking place in the blast furnace.

(iii) What is the name given to the iron obtained directly from the blast furnace? (iv) State why the iron named in 4(b)(iii) have a relatively low melting point?

(c) The following equation represents one of the reaction steps involved in the contact process: 2SO2 + O2 ⇌ 2SO3 ΔH = -395.7 kJ mo-l

(i) Why is the SO3 produced during the reaction not dissolved directly in water to form H2SO4? (ii) Why is the H2SO4 regarded as a heavy chemical? (iii) State the property exhibited by tetraoxosulphate (VI) acid in each of the following reaction equations.

Pb(NO3)2+ H2SO4→ PbSO4+ 2HNO3 (d) Write a balanced chemical equation for the reaction between propanol and sodium

Solution & Explanation:

a)(i) They are geometric/cis-trans isomers; (ii) Compound A: Trans 1, 2- dichloroethene, Compound B: Cis – 1,2- dichloroethene. (iii) The chemical properties will be the same (iv) because they both contain the same functional group; – they have same type of atoms connected in the same way

b)(1)- boiling takes place at a specific temperature; – boiling takes place throughout the liquid, – bubbles are formed vapour pressure is equal to atmospheric pressure (ii) I. It increases the boiling point of the water, ii. It lowers the boiling point of the water.

BOILING	EVAPORATION
Occurs throughout the liquid	Occurs only at the surface of the liquid
Occurs at a particular constant temperature	Takes place at all temperatures
Has no cooling effect	Has a cooling effect
Exothermic	Endothermic
Occurs using external source of heat/not spontaneous	Occurs using internal energy/spontaneous
Bubbles are observed	Bubbles are not observed
Not affected by humidity	Affected by humidity
Fast process	Slow process

Unsaturated solution	Saturated solution
Can dissolve more solute at that temperature	Cannot dissolve any more solute at that temperature
Does not contain undissolved solute at that temperature	Contains undissolved solute at that temperature

(ii) -increasing the temperature; – increasing the volume of solvent (iii.) – temperature; – volume of solvent; – nature of solute; – nature of solvent

(i) The amount of substance containing 6.02 x 10²³particles of that Substance/amount of substance containing as many particles as there are atoms in 12g of carbon 12.

(ii) n (HCI) = 25.0 x 10 dm3 x 0.1 moldm3, n (HCI) 2.5 x 10⁻³ mol.

From the equation of the reaction:

Mole of HCI = 0.025 x 0.1 = 0.0025

–>mol of MgO = 0.0025 / 2 = 0.00125mol

Molar mass of MgO = 24+ 16 =40

mass of MgO= 0.00125x 40 = 0.05g

(e) – are malleable,

-are ductile;

– are lustrous;

– have high melting point/boiling;

– have high tensile strength/hard Strong

-high density;

-solids at room temperature except mercury; –

good conductors of heat;

-good conductors of electricity: sonorous

(a) In an equilibrium reaction between gases Q and R, to form QR, the energy content of the reactants is 100 KJ and that of the product is 54 kJ. The energy content of the activated complex is 210 KJ.

(i) Draw an energy profile diagram for the reaction. (ii) Determine the: I. activation energy of the reaction il. enthalpy change ΔH of the reaction. (iii) Write a balanced equation for the reaction. (iv) Give a reason for the answer given in 3(C)(iv). (iv) State whether the reaction is exothermic or endothermic.

(b) Consider the following table:

Element	E	F	G	H
Atomic number	7	9	12	13

(i) Write the electron configuration for each of the elements. Element E, F, G, H 12 13 (ii) State: I. two elements that are metals; II. the elements(s) most likely to form an ion with a charge of +3; III. the element(s) which belong(s) to group VII. on the periodic table; IV. the formula of the compound formed between F and G. Atomic

Solution & Explanation:

(a)(i)

(ii) I.Activation = 210 – 100 = 100kj

ΔH = Hpp – Hpp = 54 – 100 = -46kj

(iii) Q22 + R22 ⇌ 2QR

(iv) The reaction is exothermic

(v) Because energy is given out ΔH is negative energy of product is lower than energy of reactant.

. IS2 2S2P3

1S2 2S2 2P5

1S2 2S2 2P6 3S2

1S2 2S2 2P6 3S2 3P I

Elements H and G

lI. Element H

III. Element F

GF2.

(c) Isotopy is simply a phenomenon whereby elements exist in various forms that have the exact same chemical properties but different atomic mass.

(d) Atoms – Ions – Molecules

(a) (i) Conc HCL and MnO2 KMnO2 (II) Conc H2SO4 (III) Downward delivery / upward displacement of air.

(ii) 16HCL + 2KMnO4 –> 2MnCl2 + 2KCl + 8H2O5Cl2

MnO2 + 41HCI –> MnCl2 2H2O2 + CI2 (III) 3Cl2 + 6NaOH –> 5NaCl + NaClO3 + 3HO2

(b) (i) – Hematite: – Coke: – Limestone (ii) C + CO2 –> C + CO2 –> 2CO Fe2O3 ⇒ Fe + 3CO2 . CaCO3 –> CaO + CO2

CaO + SiO22 –> CaSiO3

(iii) Pig Iron (iv) because it contains impurities not pure. (

c) (i) the dissolution of ŠO3in water is highly exothermic, mist of acid droplets would be formed. (ii) it is produced on a large scale: – it is required by many industries as a raw material ; it is in high demand by other industries. (iii) I. Acting as a displacement/precipitating agents; ii. Acting as an Oxidizing agent

(d) 2CH3CH2CH2OH + 2Na –> 2CH3CH2CH2ONa + H2; OR 2CH3 H7OH + 2Na –> 2CH2H7ONa + H2

4.(a)(i) Name two gases that can be used to perform the fountain experiment. (ii) What is the aim of he fountain experiment? (iii) Describe briefly the fountain experiment.

(b)(i) Name two chemical industries. (ii) State three effects of a chemical industry on the community in which it is sited.

(c)(i) Name three products of the destructive distillation of coal. (ii) Give one use each of any two of the products named in 5(c)(1).

(d)(i) Name two substances responsible for hardness in water. (ii) State two methods for the removal of hardness in water. (iii) State two advantages of hard water

Solution & Explanation:

(a)(i) ammonia: – hydrogen chloride: – sulphur (iv) Oxide (ii) To demonstrate high/extreme solubility of gases HCl / NH3 / SO2 in water.

(iii) A flask. fitted with a glass jet is filled with dry ammonia or any soluble gas. Inject water through the inlet into the flask. this dissolves the ammonia soluble gas and causes a fountain through the jet.

-a round bottomed flask is filled with gas and fitted with two delivery tubes and closed by use of clips A and B; – Invert the flask into a beaker of water: – open a clip A to allow small volume of water to enter the flask. then close it – the gas dissolves in water causing sudden reduction of pressure in the flask: open clip B: reduced gas pressure causes water from the beaker through delivery tube into the flask forming fountain.

(b) (i)-Textiles: Tannery: – Brewery Bottling company: Soaps and detergents: – food industry: pharmaceuticals: – cosmetics: petrochemical Industry: – petroleum industry; dyes: paints; agrochemicals: fertilizers; – pesticides; plastic

(ii) Improvement in the standard of living of people: – employment opportunities: – development in the community: pollution (air. land. water. etc.)

(ii) Coke (solid) fuel production of water gas extraction of metals. Coal tar; synthesis of chemicals.perfumes dyes/paints / drugs / plastics explosives / tanning of roads. Coal gas; as fuel (in homes and industries). Ammoniacal liquor;- production of fertilizer

(d)i) calcium hydrogen trioxocarbonate (IV): -calcium tetraoxosulphate (Vi): – magnesium tetraoxosulphate (Vi)

– magnesium hydrogen trioxocarbonate (IV); – iron (III) chloride.

(ii)heating the water to boiling / distillation: addition of calculated amount of calcium hydroxide: – addition of washing soda crystals: – ion exchange resin permutit

(iii)- calcium ions in the water are good for method children’s teeth and bones: it helps to reduce heart diseases; does not dissolve lead pipes: – tastes better than soft water:- provides calcium salts used in manufacturing of food in plants: production of exoskeleton of animals / shells Tannery

All your burette readings (initial and final), as well as the size of your pipette, must be recorded but no account of the experimental procedure is required. All calculations must be done in your answer booklet.

A is a solution containing 5.00 g of HNO33 in 500 cm\(^3) of solution. B is a solution of NaOH of unknown concentration.

(a) Put A into the burette and titrate it with 20.0 cm33 or 25.0 cm33 portions of B using methyl orange as an indicator. Repeat the titration to obtain concordant titre values. Tabulate your results and calculate the average volume of acid used. Equation of the reaction is HNO3(aq)3 + NaOH(aq) → NaNO3(aq)3 + H2O(l)

(b) From your results and the information provided. calculate the: (i) concentration ot A In mol dm^-3

(ii) concentration of B in mol dm-³.

(iii) concentration of B in gdm^-3

(iv) mass of NaNO33 formed. If 250 cm3 of NaOH were neutralised. [Molar mass of NaOH = 40g mol⁻¹, NaNO3 = 85 gmol⁻¹. Credit will be given for strict adherence to the instructions. for observations precisely recorded and for accurate inferences. All tests, observations and inferences must be clearly entered in this booklet, in ink, at the time they are made.

C is an organic compound. Carry out the following exercises on C. Record your observations and identity any gas(es) evolved. State the conclusions you draw from the results of each test.

(a) Put about 10 drops of C on a watch glass and ignite it using a burning splint.

(b)(i) Put about 1 cm33 of C in a test tube and add about 1 cm33 of distilled water. Shake the test tube.

(ii) Put about 1 cm3 of C in a test tube and add about 2 cm3 of acidified K2Cr2O7 solution. Warm the mixture gently and leave to stand for 5 minutes.

(c) Put few crystals of specimen D in a test tube and add about 2cm3 of C followed by about 2 cm3 of 10 % NaOH(aq) Shake the test tube vigorously.

(d) State the class of compounds to which C belongs.

Solution & Explanation:

	Test	Observation	inference
(a)	C on a water glass + burning splint	Burns with a non-Soothy/blue flame	Saturated organic compound may be present
(b)(i)	C + distilled water + shaking	One layer formed/miscible/viscous mixture/ C dissolved	C is soluble
(ii)	C + acidified K2Cr2O7(aq)7 warm and leave to stand for 5 minutes	Orange solution turns green	C is oxidized/reducing agent K2Cr2O7(aq)7 is reduced/Oxidizing agent or Primary or secondary alkanol present
(c)	D(s)(s) + C + NaOH(aq) shaking vigorously Yellow	Yellow precipitate forms with antiseptic smell	lod foam produced

(d) Alkanols

(a) A zinc salt, E when heated strongly, produced a brown gas with pungent smell, a colourless gas that rekindled a glowing splint, and a residue that was allowed to cool.

(i) identify the salt E.

(ii) Write an equation for the decomposition of E.

(iii) State what would be observed when the residue was allowed to cool.

(b) Describe how 250cm3 of 0.2 mol dm33 H2SO4 could be prepared from 150 cm3 of a 1.0 mol dm33 stock solution of the acid.

Burette reading/Titration	1st Titration	2nd Titration	3rd Titration
Final burette reading (cm33)	16.50	16.10	21.10
Initial burette reading (cm33)	0.00	0.00	5.00
Average volume of acid used	16.50	16.10	16.10

Chemistry 2020 Past Questions | WAEC

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Correct Option: Answer is C

Correct Option: Answer is C

Correct Option: Answer is C

Correct Option: Answer is B

CHEMISTRY 2020 WAEC QUESTIONS THEORY WITH SOLUTIONS

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