All 50 questions from the West African Examinations Council (WAEC) Chemistry 2013 Objective paper, with the correct answer and a full explanation for each. Free, no signup needed.
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Which of the following instruments is used in detecting the presence of radiation?
A Geiger-Muller counter is an instrument used in detecting the presence of radiation. Other devices are the diffusion cloud chamber and the scintillation counter.
The molecule which has a linear shape is
CH4 has a tetrahedral shape, NH3 has a trigonal pyramidal shape, H2S has a bent shape while CO2 has a linear shape.
The formula of the compound formed between a trivalent metal, M and a divalent non-metal, Y is
Since the metal M is trivalent, its combining power is 3. Since the non-metal Y is divalent, its combining power is 2. M2Y3 ⇒ M2Y3 (i.e. M3Y2 by cross-multiplying combining powers).
An atom of an element X gains two electrons. The symbol of the ion formed is
When atom gains electron(s), it becomes negatively charged. The magnitude of the negative charge equals the number of electrons gained. Thus, when X gains two electrons, the ion formed is X²⁻.
Which of the following statements is correct?
Atomic radius decreases across the period and increases down the group. Also, anions are larger than the parent atoms while cations are smaller than the parent atom.
Which of the following arrangements represents the correct order of electronic energy level?
The correct order of increasing energy level (Aufbau order) is 1s 2s 2p 3s 3p 4s 3d.
The element with electron configuration 1s²2s²2p⁶ 3s²3p¹ belongs to
The s.p.d.f notation is employed to tell the block an element belongs. If the electronic configuration ends in s, the element belongs to S-block; if it ends in p, the element belongs to P-block etc. Since the given electronic configuration is 1s²2s²2p⁶3s²3p¹, the element belongs to P-block. We can also tell the group and period from the s.p.d.f notation: the maximum principal quantum number is the period number while the total number of electrons in the outermost orbitals is the group number. Thus, group=3, period=3.
In the periodic table, all the elements within the same group have the same
All the elements in a particular group have the same number of electrons in their outermost shell, i.e. they have equal valence electrons.
Which of the following halogens is liquid at room temperature?
Fluorine and chlorine are gaseous while iodine is solid at room temperature; bromine is liquid at room temperature.
Rare gases are stable because they
Rare gases are stable because they have stable electronic structure. Except helium (which maintains a satisfactory duplet structure), noble gases have octet structures. They owe their chemical inactivity to the stable structures.
In the periodic table, alkaline earth metals can be found in group
Group (I) metals are also known as alkali metals while group (II) metals are also known as alkaline earth metals.
In which of the following series are the atoms arranged in order of increasing ionization energy?
Ionization energy increases across the period and decreases down the group. C, O, F, Ne (across period 2, in increasing group) is arranged in increasing ionization energy.
Which of the following bond types is responsible for the high boiling point of water?
Despite its low relative molecular mass, water boils at an unusually high temperature of 100°C. This is due to the presence of intermolecular hydrogen bonding.
In metallic solid, the force of attraction is between the mobile valence electrons and the atoms.
Each atom of a metal donates electron(s) to the electron sea. The metallic bonding actually arises from the forces of attraction between the electrons and the positively charged nuclei.
The bonds in crystalline ammonium chloride are
The three types of bonding — covalent, dative and electrovalent — are present in crystalline ammonium chloride: 1. Electrovalent/ionic: between NH4+ and Cl⁻. 2. Dative: between NH3 & H+ to make NH4+. 3. Covalent: between N & H to make NH3.
Which of the following elements is diatomic?
Sodium, iron and neon are more or less monatomic. Other diatomic elements are hydrogen, nitrogen, fluorine, chlorine and bromine.
Noble gas molecules are held together by
Noble gas molecules are held together by weak Van der Waals forces.
Which of the following statements about nuclear reaction is correct? The reaction
Nuclear reactions occur at the nucleus. They therefore involve both protons and neutrons only. Also, nuclear reactions are not affected by temperature and pressure.
Consider the reaction represented by the following equation: C2H2 + yH2 → C2H6. The value of y in the reaction is
The equation for total hydrogenation of ethyne goes as: C2H2 + 2H2 → C2H6. Comparing this to C2H2 + yH2 → C2H6, y=2.
The volume of 0.25 moldm⁻³ solution of KOH that would yield 6.5g of solid KOH on evaporation is (K = 39.0; O = 16.0; H = 1.00)
Molar mass of KOH = (39+16+1) g/mol = 56 g/mol. Mole of KOH = 6.5g/56g⁄mol = 0.1161 mol. Mole = conc(moldm⁻³)×volume(dm³). 0.1161 mol = 0.25moldm⁻³×volume. Volume = 0.1161mol/0.25moldm⁻³ = 0.4643 dm³ = 464.3 cm³.
The percentage by mass of calcium in Ca(OCl)2 is [Ca = 40.0; Cl = 35.5; O = 16.0]
Molar mass of Ca(OCl)2 = 40+2(16+35.5) = 40+2(51.5) = 40+103 = 143 g/mol. (% of calcium) = (Mass of Ca/Molar mass)×100% = (40/143)×100% = 28%.
The gas law which describes the relationship between volume and temperature is
Charles' law states that the volume of a given mass of a gas is directly proportional to the absolute temperature, provided that the pressure remains constant. V ∝ T (P = constant). V = KT, K = V/T = V1/T1 = V2/T2...
Which of the following phenomena leads to decrease in volume of a liquid in an open container?
When a container holding a liquid is left open, the volume of the liquid decreases as the liquid evaporates.
The pressure exerted by a gas is a function of the
The pressure exerted by a gas is actually due to the bombardment/collision between the gaseous molecules and the walls of the container.
Which of the following gases are arranged in increasing order of diffusion rate? [H = 1.0; C = 12; N = 14; O = 16; S = 32]
The denser a gas is, the slower its rate of diffusion — the diffusion rate is a function of the relative molecular mass. Relative molecular masses: SO2=64, O2=32, NH3=17, H2=2. Thus, the increasing order of diffusion rate will be SO2, O2, NH3, H2.
Which of the following variables is a measure of the average kinetic energy of the molecule of a gas?
The temperature is a measure of the average kinetic energy of the molecules of a gas.
When heat is absorbed during a chemical reaction, the reaction is said to be
An endothermic reaction is one during which heat is absorbed from the surrounding. The enthalpy change for an endothermic reaction is positive. The opposite applies for an exothermic reaction.
The aqueous solution which has pH > 7 is
Na2CO3 produces an alkaline solution upon hydrolysis since it is from a strong base and a weak acid.
Which of the following acids would readily react with CaCO3 to liberate CO2?
One could have expected that H2SO4 and HNO3 to react to the same extent with CaCO3 to give CO2 since they are both strong acids. However, H2SO4 will react with CaCO3 to produce CaSO4 which is only sparingly soluble. This CaSO4 can form a protective layer over the CaCO3, thereby preventing further reaction. HNO3, on the other hand, will react smoothly and flawlessly as all nitrates are soluble. CaCO3(s)+2HNO3(aq)→Ca(NO3)2(aq)+H2O(l)+CO2(g).
Which of the following compounds crystallizes without water of crystallization?
NaCl crystallizes without water of crystallization.
A substance is said to be impure if
A solid is pure if it melts at a sharp temperature or over a narrow temperature range. When a solid melts over a wide range of temperature, it is impure.
The following factors affect the solubility of a solid in a given solvent except
The solubility of a solid is affected by: i. Nature of solute ii. Nature of solvent iii. Temperature. Pressure does not have any effect on the solubility of a solid solute.
Consider the reaction represented by the equation: N2O4(g) ⇌ 2NO2(g); ΔH = +x kJmol⁻¹. What happens when the temperature is reduced at equilibrium?
Since the forward reaction is endothermic, it is favoured by increase in temperature. This means the decrease in temperature will favour backward reaction i.e production of more N2O4.
Which of the following cells produce electrical energy from chemical reactions? I. Lead-acid battery II. Dry cell III. Daniel cell IV. Electrolytic cell
Lead-acid battery, dry cell and Daniel cell are all electrochemical cells. An electrochemical cell generates electrical energy from chemical energy. An electrolytic cell does the exact opposite — it creates a chemical effect using electrical energy.
What happens at the cathode during electrolysis?
At the cathode during electrolysis, cations (positive ions) are attracted, gain electron(s) and get discharged.
Which of the following substances are electrolytes? I. PbBr2(l) II. NaCl(aq) III. NaCl(s) IV. C6H12O6(aq)
Electrolytes must be composed of ions, rather than molecules. Thus, C6H12O6 is not an electrolyte. Also, for any substance (which is already composed of ions) to serve as an electrolyte, it must be in either molten or aqueous form. Thus, NaCl(s) which is solid sodium chloride, is not an electrolyte. PbBr2(l) means molten form of lead(II) bromide, NaCl(aq) means aqueous form of sodium chloride. Both of them are fully functioning electrolytes.
Consider the redox reaction as represented by the following equation: I2(aq) + 2S2O3²⁻(aq) → 2I⁻(aq) + S4O6²⁻(aq). Which of the species in the equation is reduced?
The oxidation number of iodine changes from 0 in I2 to -1 in I⁻. This is reduction. I2 has been reduced.
The separation of petroleum fractions depends on the differences in their
The physical property harnessed in the separation of petroleum into its fractions is boiling points. The various fractions distill at different temperature, with the low-boiling fractions coming first.
The major product formed by the reaction between ethanoic acid and aqueous sodium hydroxide is
The major product formed by the reaction between ethanoic acid and aqueous sodium hydroxide is sodium ethanoate. CH3COOH + NaOH → CH3COONa + H2O (Ethanoic acid + Sodium hydroxide → Sodium ethanoate).
Which of the following organic compounds would decolourize bromine water?
The reaction with bromine water is a test for unsaturation. Since benzene is an unsaturated compound, it will react with bromine water. Alkenes and alkynes are more notable for this reaction, but benzene (despite its aromaticity being generally resistant) is given here per the printed answer key as A.
How many isomers has C3H6Cl2?
The possible isomers for C3H6Cl2 are: 1,1-dichloropropane; 2,2-dichloropropane; 1,2-dichloropropane; 1,3-dichloropropane. That is 4 isomers.
The IUPAC name of the compound (CH3)2CHCH(NH2)COOH is
The longest continuous chain has five carbon atoms with a carboxyl functional group. Thus, the parent name is pentanoic acid. Amino group (–NH2) is attached to carbon–2 while methyl is attached to carbon–4. Thus, the name is 2-amino-4-methylpentanoic acid.
Which of the following reactions is common to all hydrocarbons?
The combustion reaction is common to all hydrocarbons. All hydrocarbons burn in oxygen to yield carbon (IV) oxide and steam.
A hydrocarbon compound contains 92.3% carbon. Determine its empirical formula. [H = 1.00; C = 12.00]
C = 92.3%, H = 100% − 92.3% = 7.7%. Dividing by atomic mass: C = 92.3/12 = 7.7, H = 7.7/1 = 7.7. Dividing by smallest (7.7): C=1, H=1. Thus, the empirical formula is CH.
The main function of limestone in the blast furnace is to
The main function of limestone in the blast furnace is to remove impurity. The limestone decomposes to give calcium oxide, CaO. The calcium oxide then reacts with silica impurities to form calcium trioxosilicate (IV), known as slag. CaCO3 →(Δ) CaO+CO2; CaO+SiO2 → CaSiO3 (slag).
Which of the following raw materials is used in a plastic industry?
Ethene is the major raw material in the plastic industry.
Which of the following statements about thermoplastic material is correct? They
Unlike thermosets which harden permanently, thermoplastic materials soften and melt on heating.
Bronze is a mixture of
Bronze is an alloy of copper (Cu) and tin (Sn). Brass, on the other hand, is an alloy of copper (Cu) and zinc (Zn).
Which of the following statements about fine chemical is correct? It
Fine chemicals are produced in relatively small amount and to a greater degree of purity. Examples are dyes and drugs.
Which of the following materials is classified as a non-biodegradable pollutant?
A non-biodegradable material is one which cannot be broken down to simple harmless forms by microorganisms. Plastic is non-biodegradable.
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