All 49 questions from the West African Examinations Council (WAEC) Chemistry 2010 Objective paper, with the correct answer and a full explanation for each. Free, no signup needed.
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The electron configuration of carbon atom in its excited state is [6C]
In the ground state, the electronic configuration of 6C = 1s²2s²2p². In the excited state, an electron moves from the 2s-orbital into a 2p-orbital, so the structure becomes 1s²2s¹2px¹2py¹2pz¹.
An important medical use of nuclear radiations is in
Among other uses, radioactivity is very useful in the medical field. For instance, iodine-131 is used for treating cancer of the thyroid gland, phosphorous-32 is used for treating leukemia, cobalt-6 is useful for destroying tumor cells etc - radiotherapy.
Which of the following elements exhibits the same chemical properties as the atom ³⁵₁₇X? An element with
The chemical properties of an element depend on the atomic number (proton number), not the mass number. An element with atomic number 17 will exhibit the same chemical properties as X (which has atomic number 17).
Which of the following noble gases has electronic structure similar to that of N in NH3? [¹⁴₇N]
Nitrogen has the structure '2, 5' originally. When it combines covalently with hydrogen to form ammonia, NH3, the outermost shell now has eight electrons and the structure becomes '2, 8' just like neon.
Which of the following equations represents beta decay?
Beta decay is associated with an increase in atomic number by 1 and no change in mass number. ²³⁹₉₂U → ²³⁹₉₃Np + ⁰₋₁e correctly shows this pattern (mass number unchanged at 239, atomic number increases from 92 to 93).
The energy change which accompanies the addition of an electron to a gaseous atom is
Electron affinity is the energy change which accompanies the addition of an electron to an isolated gaseous atom.
Which of the following elements is a d-block element?
Iron (Fe) is a transition element, it is a d-block element: 26Fe = 1s²2s²2p⁶3s²3p⁶4s²3d⁶.
Calcium and magnesium belong to the same group in the periodic table because both
Elements that have equal valence electron(s) belong to the same group in the periodic table. Mg=2,8,2; Ca=2,8,8,2. Since they both have two valence electrons, they belong to group (II).
Which of the following elements is diatomic?
Iron, neon and sodium are monatomic while oxygen is diatomic. Other diatomic elements are hydrogen, nitrogen, fluorine, chlorine, bromine and iodine.
Which of the following statements about chlorine and iodine at room temperature is correct?
Chlorine is a gas while iodine is a solid at room temperature.
If X is a group III element, its oxide would be represented as
A group (III) element has a combining power of 3, while oxygen has a combining power of 2. Using these combining powers, the formula becomes X2O3.
Which of the following species correctly represents an ion of M with 13 protons and 10 electrons?
Since the atomic number (number of protons in the nucleus) of an element must remain 13, and there are fewer electrons (10) than protons (13), electrons have been lost. Thus, the element M will turn into a positive ion, 13M³⁺ (three electrons lost).
A solid substance with high melting and boiling points is likely to be a/an
Electrovalent/ionic compounds have high melting and boiling points, while covalent compounds have low melting and boiling points.
Which of the following molecules has a linear shape?
CH4 has a tetrahedral shape, CO2 has a linear shape, H2S has a bent shape while NH3 has a trigonal pyramidal shape.
Which type of bond is represented by the dotted lines in the following structure (HF...HF...HF with hydrogen bonding shown)?
The dotted lines represent intermolecular hydrogen bonding. The unbroken lines denote covalent bonding.
Which of the following molecules has a triple bond in its structure?
Nitrogen has a triple bond (N≡N) in its structure.
The bonds in crystalline ammonium chloride are
All of ionic, covalent and dative bonds are found in NH4Cl. Ionic bond: between NH4+ and Cl-, making up NH4Cl. Dative bond: between NH3 and H+, making up NH4+. Covalent bond: between N and H, making up the NH3 part of the ion.
Consider the neutralization reaction represented by the following equation: Na2CO3+2HNO3→2NaNO3+H2O+CO2. The stoichiometric ratio of acid to base is
The stoichiometric ratio of acid to base simply refers to the ratio of the mole of the acid to that of the base. Since the moles of acid in the balanced equation is 2, while that of the base is 1, the ratio is 2:1... (Note: the question asks for the ratio of acid to base as it is written; since HNO3 has coefficient 2 and Na2CO3 has coefficient 1, the ratio of acid:base is 2:1, matching option B.)
A solution of sodium trioxocarbonate (IV) contains 10.6 g in 250cm³ of solution. Calculate the concentration of the solution. [Na2CO3=106]
Mole = Mass/Molar mass = 10.6g/106gmol⁻¹ = 0.1 mol. Concentration = Mole/Volume(dm³) = 0.1mol/0.25dm³ = 0.4 moldm⁻³.
What is the volume occupied by 2 moles of ammonia at s.t.p.?
1 mole of any gas occupies a volume of 22.4dm³ at s.t.p. Thus, 2 moles of ammonia will occupy (2x22.4)dm³ = 44.8dm³ at s.t.p.
Which of the following apparatuses can be used to measure accurately a specific volume of a liquid?
Accurate measurement of volume is done with pipette or burette.
Which of the following gases can be controlled by the set-up illustrated below? [Gas → Delivery tube → Gas jar]
The set-up shows a gas being collected by downward delivery. A gas is collected by downward delivery if it is denser than air and collected by upward delivery if it is less dense than air. HCl (M.M.=36.5) is denser than air (M.M.≈28.8), so it is collected by downward delivery.
A given volume of oxygen diffused through a porous partition in 8 seconds. How long would it take the same volume of carbon (IV) oxide to diffuse under the same condition? [C=12.0, O=16.0]
Using Graham's law of diffusion: T(CO2)/T(O2) = √(RMM CO2/RMM O2). RMM O2=32, RMM CO2=44. T(CO2)/8 = √(44/32) = 1.1726. T(CO2) = 8x1.1726 = 9.381s ≈ 9.4 seconds.
Which of the following gases contains the highest number of atoms at s.t.p.?
6 moles of neon contains 6 atoms of Ne. 3 moles of O2 contains 6 atoms of oxygen. 2 moles of Cl2 contains 4 atoms of chlorine. 1 mole of C2H6 contains 2 atoms of carbon and 6 atoms of hydrogen = 8 atoms total - the highest.
Given that r is rate and p is density, the expression r ∝ 1/√p represents
The relationship r ∝ 1/√p represents Graham's law of diffusion, which states that the diffusion rate of a gas is inversely proportional to the square root of its density.
The determination of heat of combustion is carried out with A. a thermometer B. a bomb calorimeter C. an evaporating dish D. a boiling tube
The determination of heat of combustion is carried out with a bomb calorimeter.
The minimum amount of energy required for effective collisions between reacting particles is known as
The minimum amount of energy required for effective collisions between reacting particles is known as the activation energy.
Which of the following oxides is basic?
NO2 and SO2 are acidic oxides, Al2O3 is amphoteric, while CaO is basic.
Which of the following equimolar solutions would have the highest conductivity?
Given equal concentrations, ammonium trioxonitrate (V), NaNO3 and NaCl are normal salts except Al(NO3)3, FeSO4 and NaCl - conductivity increases with the total number and charge of ions produced; Al(NO3)3, releasing Al³⁺ ions with the highest charge, gives the highest conductivity among equimolar solutions.
The following substances are normal salts except
Al(NO3)3, FeSO4 and NaCl are normal salts while Mg(OH)NO3 is a basic salt.
Which of the following chlorides is insoluble in water?
AgCl is insoluble in water while KCl, NH4Cl and ZnCl2 are soluble in water.
Which of the following factors would not affect the solubility of a gas?
Nature of solvent, pressure and temperature are factors to be considered to determine the solubility of a gas. Concentration is one of them (i.e. it's not a factor affecting the solubility).
Consider the equilibrium reaction represented by the following equation: 2SO2(g)+O2(g)⇌2SO3(g); ΔH=-395.7kJmol⁻¹. Which of the following statements about the equilibrium system is correct?
Since the forward reaction is exothermic, an increase in temperature will favour the backward reaction. Since there are more moles of gases on the left hand side, a decrease in pressure will favour the backward reaction, leading to the production of more O2 (increasing its equilibrium concentration).
The rates of chemical reaction of solids are not affected by
Pressure only affects the reaction rate when gaseous reactants are involved; it does not affect reactions of solids.
Which of the statements about the cell notation Mg/Mg²⁺ //Cu²⁺/Cu is correct?
In the shorthand notation of the electrochemical cell, the compartment by the left is the anode while the compartment by the right is the cathode. Thus, magnesium is the anode.
Which of the statements about the electrolysis of CuSO4(aq) using copper cathode and platinum anode is not correct?
When platinum is used as the anode and copper is used as the cathode during electrolysis of CuSO4(aq), copper is produced at the cathode while oxygen is produced at the anode. If, on the other hand, copper is used as both anode and cathode, the copper anode dissolves while the copper cathode increases in mass - the second case is used for the purification of copper, not the platinum-anode case.
What is the change in oxidation number of manganese in the reaction represented by the following equation? MnO4⁻(aq)+8H⁺(aq)+5e⁻→Mn²⁺(aq)+4H2O(l)
The oxidation number of manganese in MnO4⁻ is +7 while its value in Mn²⁺ is +2. Thus, manganese has undergone a change of oxidation number from +7 to +2.
The quantity of electricity required to discharge 1 mole of a univalent ion is
1 mole of electrons (which equals 1 Faraday of electricity) is required to produce 1 mole of a univalent ion. 1F = 96,500 C.
Fats and oil are used as raw materials in the following industries except
Fats and oils are useful in the paint industry, margarine industry, cosmetic industry etc. The plastic industry has no use for fats and oils.
An advantage of detergent over soap is that detergents are
Detergents lather readily with water, regardless of water hardness.
The products of fermentation of sugar are
The products of fermentation of sugar are ethanol and carbon (IV) oxide: C6H12O6 → 2CO2+2C2H5OH.
The IUPAC name of C2H5COOC2H5 is
The IUPAC name of C2H5COOC2H5 is ethyl propanoate.
An organic compound contains 40.0% carbon, 6.7% hydrogen and 53.3% oxygen. What is the empirical formula of the compound? [O=16.0, C=12.0, H=1.0]
Dividing by molar mass: C: 40/12=3.3333, H: 6.7/1=6.7, O: 53.3/16=3.33125. Dividing by the smallest (3.3333): C:H:O ≈ 1:2:1. The empirical formula is CH2O.
Consider the reaction represented by the following equation: H-C≡C-H(X)→X(Y)→Y. X and Y respectively are
When ethyne undergoes stepwise hydrogenation, it first becomes ethene (X) and then ethane (Y): H-C≡C-H+H2→CH2=CH2 (ethene, X); CH2=CH2+H2→CH3CH3 (ethane, Y).
Compound N reacts with sodium metal to produce a gas that gives a 'pop' sound with a burning splint. N also reacts with ethanoic acid to give a sweet smelling liquid. Compound N is a/an
An alkanol reacts with sodium to produce hydrogen gas (which gives a 'pop' sound with a burning splint). An alkanol also reacts with alkanoic acids to give alkanoate (sweet smelling compounds).
Which of the following structures represents that of ethylethanoate?
Ethyl ethanoate has the structure CH3-C(=O)-O-CH2CH3, formed from ethanoic acid (CH3COOH) and ethanol (C2H5OH).
The main function of limestone in the blast furnace is to
The main role of limestone in the blast furnace is to remove impurities. It first decomposes thermally to give quicklime, CaO. Quicklime then combines with earthy impurities (SiO2) to give calcium trioxosilicate (IV), CaSiO3 (slag).
Which of the following substances cannot be classified as a heavy chemical?
Bleaching powder, CaOCl2, is a fine chemical, not a heavy chemical.
Which of the following metals exists as liquid at ordinary temperature?
Mercury is the only liquid metal at ordinary temperature.
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