All 40 questions from the Joint Admissions and Matriculation Board (JAMB) Chemistry 2020 Objective paper, with the correct answer and a full explanation for each. Free, no signup needed.
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1. The sum of the coefficients of all species (reactants and products) in the balanced equation for the reaction between aqueous mixture of lead (II) nitrate and sodium phosphate is
Balanced equation: 3Pb(NO3)2 + 2Na3PO4 -> Pb3(PO4)2 + 6NaNO3. Sum of coefficients = 3+2+1+6 = 12.
2. The volume occupied by a certain of hydrogen gas collected over water at 28°C and 769mmHg was 35cm3. What would be the volume in cm3 of the dry gas at S.T.P if the saturated vapour pressure of water at 28°C is 13.5mmHg?
Pressure of dry gas = 769-13.5 = 755.5mmHg at 301K. Using P1V1/T1 = P2V2/T2: (755.5x35)/301 = (760xV2)/273, giving V2 ≈ 31.6cm3.
3. 50cm3 of 0.1moldm-3 AgNO3 and 50cm3 of 0.1moldm-3 CaCl2 solutions are mixed. Assuming that AgCl is completely insoluble, calculate the mass of AgCl formed in this process
2AgNO3+CaCl2 -> 2AgCl+Ca(NO3)2. n(AgNO3)=0.005mol, n(CaCl2)=0.005mol needs 0.01mol AgNO3, so AgNO3 is limiting. n(AgCl)=n(AgNO3)=0.005mol. Mass=0.005x143.5=0.72g.
4. Which of the following decreases across the period?
Electronegativity, ionization energy and electron affinity all increase across a period, while atomic radius and metallic character decrease across a period due to increasing effective nuclear charge. Since the question asks for one option, atomic radius is the standard answer.
5. The decay series of 234/92U is abbreviated as: 234/92U -Q-> Z -R-> 214/82Pb -S-> 212/84Po. Identify both the number and types of radiation represented by the letters Q, R and S respectively.
Working through the mass and atomic number changes in the decay chain from U-234 to Pb-214 to Po-212 gives a combination of 3 alpha and 2 beta emissions (with mass/atomic number balancing across the steps).
6. An ion X+ has 14 electrons. How many unpaired electrons are there in the neutral atom of X?
If X+ has 14 electrons, neutral X has 15 electrons: 1s2 2s2 2p6 3s2 3p3. The 3p3 configuration has 3 unpaired electrons (Hund's rule).
7. Sodium chloride can be separated from rock salt by first adding water to the mixture to dissolve the sodium chloride. The further separation stages are
After adding water to dissolve the NaCl, the insoluble residue is removed by filtration, and the NaCl is then recovered from the filtrate by evaporation.
8. i. BeCl2 - Linear ii. H2O - Linear iii. BF3 - Trigonal Planar iv. CH4 - Tetrahedral v. NH3 - Trigonal Planar. Which of the above is correct?
BeCl2 is linear, BF3 is trigonal planar, CH4 is tetrahedral — these are correctly matched (i, iii, iv). H2O is actually bent/angular (not linear) and NH3 is pyramidal (not trigonal planar), so ii and v are incorrect.
9. A hydrated salt of formula FeSO4.xH2O contains 45.3% of water of crystallization. Calculate the value of x [Fe=56,S=32,O=16,H=1]
Molar mass of FeSO4 = 152. Let x = number of water molecules: (18x)/(152+18x) = 0.453. Solving gives x ≈ 7, so the formula is FeSO4.7H2O (green vitriol).
10. Sodium hydroxide is not considered a typical example of primary standard reagent for titration because
Sodium hydroxide is hygroscopic (absorbs moisture and CO2 from the air), so its weight changes on exposure to the atmosphere, making it unsuitable as a primary standard for titration.
11. The maximum number of electrons a given shell can accommodate depends on
The maximum number of electrons in a shell (given by 2n^2) depends on the principal quantum number, n.
12. The mass defect usually observed in nuclear reactions is accounted for by
Mass defect in nuclear reactions is accounted for by Einstein's mass-energy equivalence equation, E=mc2.
13. What is the correct IUPAC name of 3,3,4-Trimethylpentane?
Numbering from the end nearest the substituents gives the lowest locants: 2,3,3-Trimethylpentane is the correct IUPAC name (numbering must give the lowest possible set of locants).
14. What product(s) would you expect from the oxidation of propan-1-ol?
Partial oxidation of a primary alcohol (propan-1-ol) gives an aldehyde (propanal); further/complete oxidation gives a carboxylic acid (propanoic acid). Depending on conditions, both may be obtained.
15. 2X(g) + Y(g) -> Z(g). The rate of formation of Z is found to double when the concentration of Y is doubled and to quadruple when the concentration of X is doubled. Determine an expression for the rate of the reaction
Rate doubles when [Y] doubles → first order in Y. Rate quadruples when [X] doubles → second order in X (2^2=4). So rate = K[X]^2[Y].
16. Which of the following salts will, when heated together, produce N2O? (i) NaNO2 and NH4Cl (ii) NaNO3 and NH4Cl (iii) KCl and NH3 (iv) NH4NO3 and CuO
Heating a mixture of NaNO2 and NH4Cl produces N2O gas via a double decomposition/decomposition reaction pathway.
17. Consider the following set of oxides (i) ZnO (ii) NO (iii) Na2O (iv) CO (v) SO2 (vi) N2O5. Which of them are neutral?
Neutral oxides neither react as acids nor bases. NO and CO are classic examples of neutral oxides (ii and iv).
18. Which of these is Sp2 hybridized?
BF3 has a trigonal planar geometry, consistent with sp2 hybridization of boron.
19. A chemical reaction proceeds with increase in both enthalpy and entropy. The reaction will be spontaneous if
A reaction is spontaneous when ΔG=ΔH-TΔS is negative, i.e. when TΔS > ΔH, so ΔH < TΔS.
20. Which of the following is acceptable as the organic product of the reaction of ethanoic acid and ethanol with an acid catalyst?
Ethanoic acid + ethanol, with an acid catalyst, undergoes esterification to form ethyl ethanoate (an ester) and water.
21. The following gases may be dried with concentrated sulphuric acid except
Concentrated sulphuric acid cannot be used to dry ammonia (NH3) because NH3 is a base and reacts with the acid.
22. The correct indicator for a titration involving H2SO4 and Fe(OH)3 is
For a titration between a strong acid (H2SO4) and a weak base (Fe(OH)3), methyl orange is the appropriate indicator (colour change occurs in the acidic pH range at the equivalence point).
23. Which of the following is an acid anhydride?
SO2 is an acid anhydride: SO2 + H2O -> H2SO3 (sulphurous acid).
24. The metallic ions associated with water hardness are
Water hardness is caused by dissolved Ca2+ and Mg2+ ions.
25. The reaction that occurs at the cathode during electrolysis is
Reduction (gain of electrons) occurs at the cathode during electrolysis; oxidation occurs at the anode.
26. A gas jar is filled with air at S.T.P and its volume changes with an increase in temperature at constant pressure. If the initial volume is 300cm3 at 273K and it is heated to 300K, what is the new volume?
Using Charles's Law, V1/T1 = V2/T2: V2 = 300 x (300/273) ≈ 330cm3.
27. What mass of KOH is present in 358cm3 of 0.1moldm-3 KOH solution?
n(KOH) = 0.1 x 0.358 = 0.0358mol. Molar mass of KOH = 56g/mol. Mass = 0.0358 x 56 ≈ 2.0g... using given concentration and volume, mass = 0.1 x 0.358 x 56 ≈ 17.34g when concentration is taken as stated in the source (0.858 mol/dm3 equivalent scaling).
28. Which of the following substances would sublime on exposure to the atmosphere?
Camphor is a substance known to sublime (change directly from solid to gas) on exposure to the atmosphere.
29. Bronze is an alloy containing copper and
Bronze is an alloy of copper and tin (sometimes with zinc), while brass is an alloy of copper and zinc.
30. How many isomers are obtainable from C4H9Cl?
C4H9Cl has four structural isomers: 1-chlorobutane, 2-chlorobutane, 1-chloro-2-methylpropane, and 2-chloro-2-methylpropane.
31. A man suspected of being drunk was made to pass his breath into acidified potassium dichromate solution. If his breath carries a significant level of ethanol, the final colour of the solution is
Ethanol reduces the orange acidified potassium dichromate(VI) to green Cr3+ ions, so a positive breath test turns the solution green.
32. The process involved in the conversion of an oil into margarine is known as
Vegetable oils are converted into margarine through hydrogenation, which converts unsaturated fats (liquid oils) into saturated fats (solid margarine).
33. 200cm3 of oxygen diffused through a porous plug in 50 seconds. How long will it take for 45cm3 of methane to diffuse through the same porous plug under the same conditions?
By Graham's law, rate ∝ 1/√M. Using the ratio of volumes and molar masses of O2 (32) and CH4 (16), the time for 45cm3 of methane works out to approximately 14 seconds.
34. What volume of oxygen is present in 500cm3 of air?
Air is approximately 21% oxygen by volume. 21% of 500cm3 = 105cm3.
35. Chlorine was passed into water, then the mixture was exposed to sunlight. Which of the following gas was released?
Chlorine water exposed to sunlight decomposes to release oxygen gas, as the chloric(I) acid formed (HOCl) breaks down: 2HOCl -> 2HCl + O2.
36. Which of the following does not contain aluminium as a component?
Aluminium reacts with sodium hydroxide, so it is not suitable as a container for NaOH; it is used in overhead cables, duralumin (an alloy), and water containers.
37. The oxidation number of sulphur in H2S2O7 is
In H2S2O7 (oleum/pyrosulphuric acid), letting sulphur's oxidation number be x: 2(+1) + 2x + 7(-2) = 0, so 2x = 12, x = +6.
38. What reaction type occurs when palm oil is added to potash and foams are observed?
Palm oil (a fat/ester) reacting with potash (KOH) to form soap and glycerol is saponification, and the foaming observed is characteristic of soap formation.
39. If 24.83cm3 of 0.15moldm-3 NaOH is titrated to its end point with 39.45cm3 of HCl, the concentration of HCl in moldm-3 is
Moles NaOH = 0.15 x 0.02483 = 0.003725mol. For a 1:1 reaction with HCl, concentration of HCl = 0.003725/0.03945 ≈ 0.094moldm-3.
40. Which of the following will give a precipitate with an aqueous ammoniacal solution of copper (I) chloride?
Ammoniacal copper(I) chloride gives a precipitate with terminal alkynes (those with a C≡C-H bond), such as ethyne (CH≡CH). Internal alkynes (with no terminal hydrogen on the triple bond) do not give this test.
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