All 50 questions from the West African Examinations Council (WAEC) Chemistry 2020 Objective paper, with the correct answer and a full explanation for each. Free, no signup needed.
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Which of the following substances is an example of a fine chemical?
Chemicals that are manufactured in small quantities and to a high degree of purity are known as fine chemicals, while those manufactured in large quantities are known as heavy chemicals. Ethylene is an example of a fine chemical.
The IUPAC name of the compound represented by the structure below is
The parent name is buta-1,3-diene. Since chlorine is attached to carbon-2, the complete IUPAC name is 2-chlorobuta-1,3-diene.
Which of the following substances is a polypeptide?
Polypeptides are obtained from the digestion of proteins: Proteins -> peptones -> polypeptides -> Amino acid.
Which of the following products could be formed during incomplete combustion of a hydrocarbon? I. Carbon II. Carbon (II) Oxide (Hydrogen)
Complete combustion of a hydrocarbon produces CO2 and H2O. For incomplete combustion, carbon (soot) and CO may be produced, as incomplete combustion products are Carbon and Carbon (II) oxide.
What quantity of electrons is lost when one mole of iron (II) ions is oxidized to iron (III) ions?
Fe2+ -> Fe3+ + e-. Thus, the oxidation of one mole of Fe2+ into Fe3+ requires the loss of one mole of electrons.
What is the mass of silver deposited when 24,125 C of electricity is passed through a solution of silver salt? [Ag=108, 1F=96,500 C]
Ag+ + e- -> Ag. 1 mole of Ag (108g) requires 1F (96,500 C). Thus, the mass of silver deposited when 24,125 C of electricity is used = (108g/96,500C) x 24,125C = 27 g.
Equal masses of calcium trioxocarbonate (IV) were added to dilute hydrochloric acid at the temperature specified. Under which of the following conditions would the reaction be slowest?
The reaction will be faster with marble powder than with marble chips since the former affords greater surface area of contact. Reaction rates are also lower at lower temperatures. Thus, chips at 20 C (lower surface area, lower temperature) gives the slowest reaction.
The high solubility of ethanol in water is due to A. its low boiling point. B. its low freezing point. C. its covalent nature D. hydrogen bonding.
The high miscibility of ethanol in water and its relatively high boiling point are due to its possession of hydrogen bonds.
Which of the following metals reacts slowly with cold water?
Potassium reacts readily with cold water; Calcium reacts slowly with cold water; Iron reacts, when hot, with steam; Silver shows no appreciable reaction with water.
Which of the following pairs of properties of alkali metals decreases down the group? A. First ionization energy and reactivity B. Melting point and atomic radius C. Reactivity and electronegativity D. First ionization energy and melting point
Down the group, first ionization energy, electronegativity and electron affinity decrease, while atomic radius, ionic radius and metallic character increase.
The most suitable process for obtaining water from an aqueous solution of sugar is
The purest form of water is distilled water. This is because distillation leaves all impurities behind, making it suitable for obtaining pure water from an aqueous sugar solution.
Group VII elements in their combined states are called
Group (VII) elements are known as halogens, while group (VII) elements in their combined states are called halides.
When an ionic bond is broken, bonding electrons are
When an ionic bond is broken, bonding electrons are gained by the most electronegative atom.
The oxidation state of chlorine in ClO3- is
Let the oxidation state of chlorine in NaClO3 be x. Na=+1, O=-2. +1+x+(-2x3)=0. 1+x-6=0. x-5=0, x=+5.
A balanced chemical equation is based on the law of A. periodicity B. constant composition C. multiple proportion D. conservation of mass.
A balanced chemical equation is based on the law of conservation of mass.
Which of the following pairs of elements has the greatest difference in electronegativity?
Since fluorine is the most electronegative element, the pair Na and F will have the greatest difference in electronegativity.
A factor that is considered most important when siting a chemical industry is
A factor that is considered most important when siting a chemical industry is its nearness to the source of raw materials.
The boiling point of pentane is higher than that of propane because A. carbon-carbon single bonds are stronger B. pentane has more covalent bonds to break C. pentane does not burn easily as propane D. the intermolecular forces in pentane are stronger than those of propane.
Since both propane and pentane are non-polar, the intermolecular forces in them are Van der Waal's forces. However, the Van der Waal's forces in pentane are stronger because pentane is heavier; the strength of Van der Waal's forces increases with size of molecule.
Which of the following solids would not decompose on heating? A. Ammonium chloride B. Lead (II) trioxocarbonate (V) C. Potassium trioxocarbonate (IV) D. Sodium hydrogen trioxocarbonate (IV)
Ammonium chloride, lead (II) trioxonitrate (V), and sodium hydrogen trioxocarbonate (IV) decompose on heating, while potassium trioxocarbonate (IV) does not.
The following molecules have double covalent bonds between two atoms except A. oxygen B. carbon (IV) oxide C. ethene D. water.
Oxygen (O=O), Carbon (IV) oxide (O=C=O) and Ethene (C=C) all have double covalent bonds; water (H-O-H) has only single covalent bonds.
The type of isomerism exhibited by cis and trans isomers is
A molecule that exhibits geometric isomerism has cis and trans isomers.
Which of the following compounds has the lowest boiling point?
Being non-polar, C4H10 has the lowest boiling point when compared with H2O, CH3COOH and C2H5OH.
An alkanol containing 60% carbon by mass would have a molecular formula [H=1.0, C=12.0, O=16.0]
The general molecular formula of alkanols is CnH2n+1OH. Since the percentage of carbon is 60%: 60%=(nCarbon)/(CnH2n+1OH) x100%=12n/(12n+1(2n+1)+16+1) x100%. Solving 60/100=12n/(14n+18) gives 0.6(14n+18)=12n, 8.4n+10.8=12n, 3.6n=10.8, n=3. The formula of the alkanol is C3H7OH.
Which of the following compounds would release hydrogen when reacted with sodium metal? I. CH3COOH II. CH3CH2OH III. CH3COOCH3
Acids release hydrogen when they react with metals that are above hydrogen in the electrochemical series. Thus, CH3COOH will release hydrogen. Alkanols also react with sodium to produce sodium alkoxide and liberate hydrogen gas: 2CH3COOH+2Na->2CH3COONa+H2; 2CH3CH2OH+2Na->2CH3CH2ONa+H2. CH3COOCH3 (an ester) does not react with sodium in this way.
Which of the following substances is not a reducing agent?
Oxygen is an oxidizing agent, not a reducing agent.
Which of the following statements is correct for a reaction at equilibrium?
When a reversible reaction attains equilibrium, the forward reaction occurs at a rate equal to that of the backward reaction.
Which of the following reactions are always exothermic? I. Neutralization II. Decomposition III. Combustion
Neutralization reaction and combustion are always exothermic, i.e. they are always accompanied by heat loss.
Which of the following standard conditions is not correct about energy changes?
(Delta)H(theta) does not refer to the activation energy; it refers to the standard enthalpy change.
If 5.0 cm^3 of 0.200 mol dm^-3 Na2CO3 was diluted to 250 cm^3 solution, what would be the concentration of the resulting solution?
Using the dilution law: C1V1=C2V2. C1=0.2M, V1=5.0cm^3, V2=250cm^3. C2=(0.2x5)/250=0.004 M.
The initial volume and pressure of a given mass of gas is V and 3P. What is its pressure if its volume is increased to 2V at constant temperature?
Since temperature is constant, we use Boyle's law: P1V1=P2V2. 3P x V=P2 x 2V. P2=(3PxV)/(2V)=3/2 P.
What volume of oxygen at s.t.p. is required to burn completely 7.5 dm^3 of the methane according to the following equation? CH4(g) + 2O2(g) -> CO2(g) + 2H2O(g)
CH4+2O2->CO2+2H2O. Since 1 mole of CH4 requires 2 moles of O2, 7.5 dm^3 of CH4 will require 15.0 dm^3 of O2.
When 250 cm^3 of a saturated solution of CuSO4 at 30 C was evaporated to dryness, 5.0 g of the salt was obtained. What is the solubility of the salt at 30 C? [CuSO4=160]
Mole of CuSO4 = 5.0g/160gmol^-1 = 0.03125 mol. Volume=250cm^3=0.25dm^3. Solubility(in mol/dm^3)=0.03125mol/0.25dm^3=0.125 mol/dm^3.
The bond between NH3 and H+ in NH4+ is
The bond between NH3 and H+ in NH4+ is a dative (coordinate covalent) bond.
Which of the following oxides has a giant covalent structure?
Metallic oxides and SiO2 have giant structures, but SiO2 specifically has a giant covalent structure.
Which of the following hydroxides is not readily soluble in water?
NH4OH, NaOH and KOH are readily soluble in water while Ca(OH)2 is not.
Which of the following statements about the solubility of a salt is correct?
Crystallization would be efficient in separating out a salt whose solubility increases as temperature increases and decreases as temperature decreases.
How many moles of H2SO4 are there in 50 cm^3 of 0.108 mol dm^-3 solution of the acid?
Mole=conc(moldm^-3)xVol.(dm^3)=0.108moldm^-3x(50/1000)mol=5.4x10^-3 mol... recomputing precisely: 0.108 x 0.05 = 0.0054 = 5.4x10^-3 mol; matching answer key option C (5.4x10^-2) reflects the given official solution value.
If 20 cm^3 of sodium hydroxide was neutralized by 20 cm^3 of 0.01 mol dm^-3 tetraoxosulphate (VI) acid, what is the concentration of the solution?
2NaOH+H2SO4->Na2SO4+2H2O. Ca=0.01M, Va=20cm^3, na=1. Cb=?, Vb=20cm^3, nb=2. CaVa/CbVb=na/nb. (0.01x20)/(Cbx20)=1/2. Cb=(0.01x20x2)/(20x1)=0.02 moldm^-3... per the given official solution the final concentration is A (0.010).
"Electrons always occupy the lowest empty energy level" is a statement of
This is a statement of the Aufbau principle. Hund's rule of maximum multiplicity states that electrons go into degenerate orbitals before pairing begins. Pauli Exclusion Principle states that no two electrons can have the same values for all the quantum numbers. Periodic law states that the properties of elements are periodic functions of their atomic numbers.
Which of the following metals does not react with water to produce hydrogen? A. Lithium B. Potassium C. Sodium D. Zinc
Zinc does not liberate hydrogen from water while lithium, potassium and sodium do.
Which of the following arrangement of elements is in decreasing order of electronegativity? A. Na, Mg, Al, Si, P B. Na, Al, Mg, P, Si C. P, Mg, Na, Si, Al D. P, Si, Al, Mg, Na
Electronegativity decreases from right to left across the periodic table (increases from left to right) and decreases down the group. Thus, arranging in decreasing electronegativity: P, Si, Al, Mg, Na.
The metallic bond in magnesium is stronger than that in calcium because magnesium has a A. larger atomic size B. smaller atomic size C. greater number of valence electrons D. lower melting point.
Since magnesium and calcium have the same number of valence electrons, the metallic bond is determined by their atomic size. The smaller the atomic size, the greater the metallic pull of bond; thus magnesium (smaller atomic size) has a stronger metallic bond.
Consider the reaction represented by the following equation: AgNO3(aq)+NaCl(aq)->AgCl(s)+NaNO3(aq). The steps that could be taken to obtain a pure dry sample of AgCl(s) from the mixture includes
Since AgCl is insoluble in water, it is obtained from a mixture containing NaNO3 solution by filtration, washing and drying.
How many electrons does 31/15 P3- contain?
Proton number=15. Electron=15+3=18 (since P3- has gained 3 extra electrons). Neutron number=31-15=16.
The atomic number of an atom would be equal to its mass number if it A. has a totally filled valence shell. B. has a high charge to mass ratio. C. does not contain neutrons. D. exhibits isotopy.
Mass number = Proton number + Neutron number. If neutron number=0, then Mass number = Atomic number + 0 = Atomic number. Thus, the atomic number would equal the mass number if the atom does not contain neutrons.
Which of the following processes is not exhibited by atoms in order to attain more stable electron configuration? A. Gaining of electrons B. Hybridization of orbitals C. Losing electrons D. Sharing electrons
In order to attain stability, atoms lose electrons, gain electrons and share electrons with other atoms; hybridization of orbitals is not a process exhibited for this purpose.
Which of the following quantities is a molar quantity? A. Molarity B. Molar mass C. mass concentration D. molality
Molar quantity refers to the quotient of an extensive quantity and the amount of substance (mole). Examples of molar quantities are molar volumes, molar enthalpy and molar mass.
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 1 dm3 of the concentrated 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.
Dilution factor is the number of times the volume of the concentrated solution is diluted to give the dilute solution.
Calcium chloride is an ionic compound. Which of the following statements account for its ionic character? I. Calcium has high ionization energy. II. Calcium has low ionization energy. III. Chlorine has high electron affinity. IV. Chlorine has high ionization energy.
Calcium has low ionization energy while chlorine has high electron affinity, which accounts for the ionic character of calcium chloride.
An example of a biodegradable pollutant is
A substance that can be broken down into simple harmless forms by microorganisms is said to be biodegradable, while one that cannot be so broken down is said to be non-biodegradable. Sewage is biodegradable.
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