All 50 questions from the West African Examinations Council (WAEC) Chemistry 2019 Objective paper, with the correct answer and a full explanation for each. Free, no signup needed.
Advertisement
The periodic table is an arrangement of elements according to their
The periodic table arranges elements according to increasing atomic number; the electron arrangement (K,L,M,N notation) then fixes each element's group and period.
A solid substance with high melting and boiling points is likely to be
Electrovalent (ionic) compounds have strong electrostatic forces between ions, giving them high melting and boiling points, unlike covalent compounds which generally have low melting and boiling points.
Which of the following metals reacts slowly with cold water?
Iron reacts only slowly with cold water (it reacts readily only with steam when hot); silver does not react at all, while calcium and potassium react vigorously with cold water.
Which of the following elements has the highest ionization energy?
Fluorine, being at the top right of the periodic table (small atomic radius, high nuclear charge, less electron shielding), has the highest first ionization energy of the four.
The bleaching action of chlorine is through the process of
Chlorine bleaches by oxidation: it forms HOCl in water, which oxidizes coloured substances to colourless ones.
If 6.5 g of magnesium ribbon reacts completely with hydrochloric acid, how many grams of hydrogen gas would be liberated? Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g) [H=1.0, Mg=24.0]
Moles of Mg = 6.5/24 = 0.2708 mol. Mole ratio Mg:H2 = 1:1, so moles of H2 = 0.2708 mol. Mass of H2 = 0.2708 × 2 = 0.54 g.
Calcium and magnesium belong to the same group of the periodic table because both
Elements in the same group have the same number of valence (outermost) electrons. Ca (2,8,8,2) and Mg (2,8,2) both have 2 valence electrons, placing them in Group II.
Which of the following substances is non-polar?
Br2 is a homonuclear diatomic molecule with a symmetric, non-polar covalent bond, whereas NH3, H2O and HBr are all polar molecules.
Which of the following pairs of compounds have the same molecular mass? [H=1.0, C=12.0, O=16.0]
Carbon(II) oxide (CO) = 12+16 = 28. Ethyne (C2H2) = 24+2 = 26. Actually the matching pair is Carbon(II) oxide (28) and Ethene (C2H4 = 28); both equal 28 g/mol.
When an equilibrium is established between dissolved and undissolved solutes, the solution is said to be
A saturated solution is one in which a dynamic equilibrium exists between the dissolved solute and the undissolved (excess) solute.
How many moles of carbon(IV) oxide contain 16.0 g of oxygen? [C=12.0, O=16.0]
1 mole of CO2 contains 32 g of oxygen (2 × 16). Therefore 16 g of oxygen is contained in 0.5 mole of CO2.
Stoichiometry is based on the law of
Stoichiometric calculations rely on balanced equations, which in turn obey the law of conservation of mass (mass of reactants = mass of products).
What is the percentage by mass of hydrogen in CH3COOH? [CH3COOH = 60]
%H = (1×4/60) × 100% = 6.6667% ≈ 6.7% (there are 4 hydrogen atoms in CH3COOH).
Which of the following compounds has coordinate bonds in its structure?
In Al2Cl6 (the dimer of AlCl3), two AlCl3 units are linked together through coordinate (dative) covalent bonds formed by chlorine atoms donating lone pairs.
The name of the compound CH3C(CH3)2CH2CH2CH3 is
The longest chain has 5 carbons (pentane) with two methyl groups on carbon 2, giving the IUPAC name 2,2-dimethylpentane.
The properties of a good primary standard include the following except
A good primary standard should have a HIGH molecular mass (to reduce weighing error), so 'low molecular mass' is the property that does NOT apply.
What is the solubility of a salt X at 25°C if the saturated solution of the salt contains 0.28 g in 100 cm³ of solution? [X = 56.0 g]
Moles of X = 0.28/56 = 0.005 mol in 100 cm³ = 0.1 dm³. Solubility = 0.005/0.1 = 0.05 moldm⁻³.
Vinegar is an aqueous solution of
Vinegar is a dilute aqueous solution of ethanoic acid (acetic acid, CH3COOH).
Which of the following solutions containing one mole per dm³ have the lowest pH?
Hydrochloric acid is a strong acid and fully ionizes, giving the highest H+ concentration and therefore the lowest pH among the options.
A sample of air was bubbled into water. The pH of the water slowly changed from 7 to 6. Which of the following gases in the sample caused the change?
A drop in pH (7→6) indicates the gas is acidic in water. Carbon(IV) oxide (CO2) forms a weakly acidic solution (carbonic acid); CO, N2 and O2 are neutral in water.
The most suitable indicator for the reaction between NaOH(aq) and HCOOH(aq) is
This is a weak acid (methanoic acid) vs strong base (NaOH) titration; phenolphthalein (colour change in the alkaline range) is the suitable indicator.
Consider the following reaction equation: 2Al(OH)3+3H2SO4→Al2(SO4)3+6H2O. How many moles of tetraoxosulphate(VI) acid is needed to produce 0.5 mol of aluminium tetraoxosulphate(VI)?
From the equation, 1 mole of Al2(SO4)3 requires 3 moles of H2SO4. Therefore 0.5 mol of Al2(SO4)3 requires (3/1) × 0.5 = 1.5 moles of H2SO4.
Which of the following pairs of substances has the highest heat of neutralization?
A strong acid reacting with a strong base gives the greatest (most negative) heat of neutralization, about –57.3 kJ/mol, regardless of the specific acid/base used.
When quick lime dissolves in water,
The reaction of quicklime (CaO) with water to form slaked lime, Ca(OH)2, is exothermic, so heat is evolved.
If the value of ΔH is positive for a reaction, it means that the reaction is
A positive ΔH means the products have more energy than the reactants, i.e. the reaction absorbs heat — an endothermic reaction.
Which of the following compounds is formed by the oxidation of ethanol?
Partial oxidation of ethanol (CH3CH2OH) gives ethanal, and total/further oxidation gives ethanoic acid, CH3COOH (CH3CO2H).
What is the characteristic of the gas liberated when dilute hydrochloric acid is added to iron(II) sulphide?
The gas produced is hydrogen sulphide (H2S), which has a rotten-egg smell and turns lead(II) ethanoate paper black due to formation of black lead(II) sulphide.
An organic compound contains 72% carbon, 12% hydrogen and 16% oxygen by mass. What is the empirical formula of the compound? [H=1.0, C=12.0, O=16.0]
Moles: C=72/12=6, H=12/1=12, O=16/16=1. Ratio C:H:O = 6:12:1, giving empirical formula C6H12O.
When excess chlorine is mixed with ethene at room temperature, the product formed is
Chlorine adds across the C=C double bond of ethene (addition reaction) to form 1,2-dichloroethane, C2H4Cl2.
Which of the following reagents could be used to distinguish between propan-1-ol and propan-2-ol?
Propan-2-ol (a secondary alcohol) gives a positive iodoform test with I2/NaOH (yellow precipitate), while propan-1-ol does not, since iodoform reaction requires a CH3CH(OH)- structure.
If a catalyst is added to a system at equilibrium and the temperature and pressure remain constant, there would be no effect on the
A catalyst speeds up both the forward and reverse reactions equally (by lowering activation energy) and does not alter the position of equilibrium or the heat of reaction.
Consider the following reaction equation: 3Cl2+2NH3→N2+6HCl. NH3 is acting as
The oxidation number of nitrogen increases from –3 in NH3 to 0 in N2, meaning NH3 is oxidized; a substance that is oxidized acts as the reducing agent.
The oxidation number of chromium in Cr2O7²⁻ is
Let Cr = x: 2x + (7×–2) = –2 → 2x − 14 = –2 → 2x = 12 → x = +6.
The process by which alkanoic acid reacts reversibly with alkanols is known as
Esterification is the reversible reaction of a carboxylic (alkanoic) acid with an alcohol (alkanol), usually with a mineral acid catalyst, to form an ester and water.
If the oxidation potential of Mg and Al are +2.37 volts and +1.66 volts respectively, then Mg would not
Since Mg has a higher (more positive) oxidation potential than Al, Mg is more reactive and will displace Al from solution; therefore Mg would NOT be replaced by Al.
Consider the reaction represented by the following equation: 2SO2(g)+O2(g)↔2SO3(g). A decrease in pressure of the reaction would
There are more gas moles on the left (3 mol) than the right (2 mol). A decrease in pressure favours the side with more moles of gas, shifting equilibrium to the left (the backward reaction), in line with Le Chatelier's principle.
In a mixture of gases which do not react chemically together, the pressure of the individual gas is
The pressure exerted by one gas in a mixture of non-reacting gases is called its partial pressure; the sum of all partial pressures gives the total pressure.
Biotechnology is applied in the
Biotechnology (using micro-organisms) is widely applied in the biological treatment of sewage.
Which of the following organic compounds has the lowest boiling point?
C4H10 and (CH3)3CH (2-methylpropane) both have RMM 58, but the more highly branched (CH3)3CH has a lower boiling point than straight-chain C4H10, since branching reduces intermolecular contact/van der Waals forces.
Consider the following reaction equation: C2H2 –(H2)→ X –(H2)→ Y. X and Y respectively are
Ethyne (C2H2) is hydrogenated to ethene (X), which is further hydrogenated to ethane (Y): C2H2→C2H4→C2H6.
Which of the following substances can be classified as heavy chemical?
Heavy chemicals are those manufactured in large quantities (e.g. H2SO4), while fine chemicals are produced in small quantities but to a high degree of purity (e.g. dyes).
In Boyle's law,
Boyle's law states that at constant temperature, the volume of a fixed mass of gas is inversely proportional to its pressure.
Which of the following apparatus is not used in volumetric analysis?
A desiccator is used to dry substances or protect them from atmospheric moisture; it plays no role in volumetric (titration) analysis, unlike the pipette, burette and conical flask.
One of the deductions from Rutherford's alpha particle scattering experiment is that
Rutherford's gold-foil experiment showed that atoms have a small, dense, positively charged nucleus, from which it was deduced that the nucleus (and its protons) is positively charged.
The atoms 64/29Cu and 65/30Zn have the same number of
For 64Cu: neutrons = 64 − 29 = 35. For 65Zn: neutrons = 65 − 30 = 35. Both isotopes therefore have the same number of neutrons (35).
The number of electrons in the 3d orbital of 24Cr is
Chromium is an exception to the normal filling order: its actual configuration is 1s²2s²2p⁶3s²3p⁶4s¹3d⁵ (not 3d⁴), giving 5 electrons in the 3d orbital.
The method used to collect a gas in the laboratory depends on its I. boiling point II. Density III. Smell IV. Solubility in water.
The method of gas collection depends on the gas's density (for upward/downward delivery) and its solubility in water (whether it can be collected over water) — i.e. II and IV only.
Which of the following methods is not used for the separation of mixtures?
Chromatography, crystallization and distillation are physical separation techniques; electrolysis is a chemical decomposition process, not a separation technique for mixtures.
The group to which elements belong in the periodic table is determined by the number of
An element's group in the periodic table is determined by its number of valence (outermost shell) electrons; the period is determined by the number of shells.
Kipp's apparatus can be used to prepare
Kipp's apparatus (Kipp generator) is used for the intermittent, small-scale supply of gases such as hydrogen sulphide (H2S) and carbon(IV) oxide from a solid and a liquid reagent.
Advertisement
…and/or go further with premium services and No Ads.