1978 JAMB CHEMISTRY ANSWERS

UME 1978 CHEMISTRY ANSWERS

1.     All scientific laws have a constant. Options C & D would have been correct for Boyle’s and Charles’ laws if there was temperature and pressure constants respectively. However, according to the kinetic theory of gas the average kinetic energy of gas is directly proportional to its absolute temperature

                                            [A]

2.     2% of an aqueous solution means 2g of the solute in 100cm³ of the solvent 1000cm³ of the solvent will contain  x  = 20g of the solute

Molecular mass of H₂SO₄ = 98g

Molarity =  = 0.2mol/dm³

[N.C.O: NO CORRECT OPTION]

3.     Zinc oxide can react with both acid and base, thus it is said to be amphoteric. Other amphoteric oxides are oxides of Aluminum, Lead and Tin. The mnemonic for amphoteric oxide is ZALT.      [C]

4.     Sodium reacts vigorously with water with evolution of heat which makes the reaction vessel feel hotter when touched. Thus, its reaction with water is said to be exothermic                                          [D]                      

5.     The periodic law states that the property of an element is the periodic function of its atomic numbers. Thus, elements are arranged in the periodic table according to their atomic numbers                                        [D]

6.     Equation for the reaction:

 H₂SO₄ + 2NaOH Na₂SO4 + H₂O

Mole ratio of the acid to base = 1: 2

 =  Substitute:

 =  

Vb =  = 50cm³                [E]

7.     Ammonium chloride when heated changes directly from solid to gas. When the gaseous constituents are cooled it changes back to solid without passing through a liquid state. This process is called sublimation. Other substances that sublime are iodine, naphthalene, camphor, etc.         [B]

8.     Elements with a large difference in their number of valence electrons or electro negativity values often form an electrovalent bond while elements with a small difference in their valence electrons or electronegativity values tend to form a covalent bond. Q and S has the valence electrons of 1 and 7 respectively. The difference between them is (7-1) = 6 which is large, thus they all form an electrovalent bond            [D]

9.     In XCl₄ : 1 atom of X combined  with 4 atoms of chlorine.

In XCl₃ : 1 atom of X combined with 3 atomsof chlorine.

In XCl₂ : 1 atom of X combined with 2- atoms of chlorine.

The various atoms of chlorine that combined with a fixed mass of X are in simple multiple ratio of 4:3:2. This is the law of multiple proportion                               [A].

10.  Formula of the compound = KClO₃

Sum of the oxidation number of elements in a compound = 0

Thus, K + Cl + 3 [O] = 0

1 + Cl + 3 (-2) = 0

1 + Cl – 6 = 0

Cl – 5 = 0

Cl = +5                                    [D]

11.  When the air passes through alkaline pyrogallol, oxygen and carbon (IV) oxide are absorbed. On passing through a quicklime, water vapour is absorbed. The residual gases are therefore nitrogen and rare gases [D]

12.  Endothermic reaction is a reaction whereby heat energy is absorbed from the surroundings eg the formation of water gas     [D]

13.  Due to the great demand for certain fractions of petroleum, large hydrocarbons such as kerosene, diesel, etc. are broken down into smaller hydrocarbons like petrol. This process is called cracking [C]

14.  pH scale consists of a whole number values ranging from 1-14. The midpoint of the pH scale is 7 and this indicates neutrality. Acidity of a solution increases as the pH decreases while the basicity of a solution increases as the pH increases [D]

15.  Nitrates of sodium and potassium when heated decomposes into their respective dioxonitrates and oxygen. Nitrates of Magnesium, calcium, zinc iron, lead and copper when heated decompose into their respective metallic oxides, oxygen and nitrogen(IV)oxide (C)

16.  In the electrochemical series all the elements: sodium, Magnesium, Iron and Calcium are above hydrogen except copper which is lower in the series. Elements that are higher in electrochemical series are said to be more electropositive and only a more electropositive element can displace the other from its solution (E)

17.  Calcium tetraoxosulphate(VI) causes permanent water hardness while calcium hydrogen trioxocarbonate causes temporary water hardness boiling or addition of calcium hydroxide removes only temporary and permanent water hardness can be simultaneously removed using sodium trioxocarbonates (D)

18.  Cations present in CuSO4 solution Cu²⁺ and H⁺. Amions present SO₄^2- and OH^-. During electrolysis the cations – Cu²⁺ and H⁺ migrate to the cathode where Cu²⁺ is preferentially discharged. The anions: SO₄^2- and OH^- migrate towards the anode where OH^- is preferentially discharged to liberate oxygen. The H⁺ and SO₄^2- present in the solution associate to form H₂SO₄ which turns blue litmus red (D)

19.  Alkanoic (carboxylic) acid reacts reversibly with alkanol in the presence of concentrated tetraoxosulphate(VI)acid as a catalyst to  form an ester. The process is called esterification (C)

20.  The compound – CH₃COOH has the functional group – COOH which belongs to the homologous series alkanoic acid. No of carbon atoms = 2. Thus, the name is ethanoic acid. The compound CH₂ = CH₂ has a double bond which is a feature of the homologous series –alkene. No of carbon atom = 2. Thus the name is ethene (B).

21.  From Graham’s Law of diffusion heavier gas takes a greater time to diffuse. Chlorine being heavier than oxygen will take a longer time to diffuse.

Applying Graham’s law:

=    =  

T² of Cl₂ =   = 108.4

T =   = 10.4

If 30cm3 of Cl2 diffused in 10.4 seconds,

60cm3 of Chlorine will diffuse in  ×

= 20.8 21 seconds            [C]

22.  Brass is an alloy of copper and zinc while bronze is an alloy of copper and tin. Thus the element common to both is copper [C]

23.  Ammonia cannot to dried with concentrated tetraoxosulphate because the acid reacts with the gas to form ammonium tetraoxosulphate(VI)

2NH₃ + H₂SO₄ (NH₄)₃SO₄.

Thus, suitable drying agents for ammonia are calcium oxide and silica gel which do NOT react with the gas [E]

24.  The 3 raw materials used in the blast furnace are coke, iron (III) oxide and calcium trioxocarbonate. During smelting process, coke is oxidized to carbon(IV)oxide at the bottom of the furnace. At the upper part of the furnace, the CO₂ is reduced by coke to CO which reduces the Fe₂O₃ to metallic iron. The CaCO3 decomposes to yield CaO which then combines with SO₂ (an impurity to form CaSiO₃. Thus, there is no oxidation of iron in the blast furnace [C]

25.  The homologous series – alkane has a general molecular formula of C_nH_2n+2. Comparing this with the compound C₂₄H₄₈

n = 24. If n = 24, the expected number of H-atoms for alkane = 2n + 2 = 2(24) +2 = 50 Since, the number of atoms of H in the compound is 48, the compound does not belong to the homologous series – alkane [E]

26.  In the formation of ammonium ion, the lone pair of electron shared between ammonia molecule and hydrogen is donated by the ammonia molecule. This type of bond is called co-ordinate (dative) covalent bond [D]

27.  Alkanol (alcohol) is a homologous series with the functional group – OH which is only present in the compound CH₃OH. The IUPAC name of the compound is methanol.

28.  Among the 3-atomic particles namely proton, neutron and electron. Only protons an neutrons occupy the nucleus of the atom while electrons are distributed round the shells (E).

29.  Hypochlorous acid also known as oxochlorate(I)acid (HOCl) is formed from dissolution of chlorine in water. It decomposes easily to yield hydrochloric acid and oxygen. The released oxygen causes bleaching by oxidizing the material (C).

30.  Boiling occurs when the vapour pressure of a liquid equals the atmospheric pressure. Since atmospheric pressure decreases with height, liquids would boil at lower temperature at places of  high attitudes like mountains (A)

31.  A substance that can absorb large amount of water to form a solution when exposed to the atmosphere is said to be deliquescent. Deliquescent substances include: NaOH, KOH, CaCl₂, FeCl₃ MgCl₂, P₂O₅ (D)

32.  Alkanes are saturated hydrocarbons due to the presence of single bond. Saturated compounds cannot undergo addition reaction but substitution reaction eg CH₄ +Cl₂ CH₃Cl + HCl (A)

33.  In exothermic reaction, the forward reaction is favoured by decrease in temperature. Thus, reducing the temperature of the exothermic reaction from 800^oC to 500^oC will favour the forward reaction by increasing the concentration of SO₃ (C)

34.  Zinc when added to a bluish green solution of CUSO₄ displace the Cu from its solution to form a colourless ZnSO₄ and Cu metal. This is due to the more electropositive nature of Zn than Cu (C)

35.  BaCl₂ solution when added to acidified SO₄^2- forms a while precipitate of BaSO₄ insoluble in excess dilute acid. This is a confirmatory test for tetraoxosulphate (VI) (A)

36.  Acetylene (ethyne) undergoes substitution reaction with ammoniacal solution of copper (I) chloride and ammoniacal solution of silver trioxonitrate to form copper(I)carbide and silver carbide respectively. Ethene does not undergo substitution reaction. Option C is wrong because the AgNo₃ does NOT contain ammonia                                          [D]

37.  In the forward reaction, Cl₂ Oxidizes Fe²⁺ to F³⁺. In the backward reaction, Fe³⁺ oxidzes Cl^- to Cl₂. But Fe³⁺ is an ion while Cl₂ is NOT. [C]

38.  Hydrogen sulphide reduces iron (III) chloride to a pale green solution of iron (II) chloride with the formation of a yellow deposit of sulphur and hydrochloric acid

2FeCl₃ + H₂S 2FeCl₂ + 2HCl +S       [E]

39.  A Physical change is easily reversible and there is no change in the chemical compositions of the substance formed. After cooling water to ice, the water can be obtained back by melting the ice [D]

40.  Filtration is used to separate soluble substances from insoluble substances. Potassium nitrate, lead nitrate, glucose and sodium chloride are all soluble in water. Starch is insoluble in water. Starch is insoluble in cold water but soluble in warm water. [D]

41.  Atomic masses of W and Y are (16 + 16) = 32 and (19 + 20) = 39 respectively. These values are greater than 40 Y in addition has an odd atomic no of 19 and forms a unipositive ion due to the valency of 1 i.e (2, 8, 8, (1) [D]

42.  Among the options provided, only trioxotrates of zinc and lead can give off the brown gas (NO₂) when heated. However Zn²⁺ is soluble in excess ammonium hydroxide while Pb²⁺ is insoluble in excess ammonium hydroxide [B]

43.  From the graph, at 0⁰C:

Solubility of X = 175g/dm3

Solubility of Y = 83.3g/dm3

Solubility of Z = 25g/dm3

At 100⁰C, all the solutions contain 200g of the solute.

On cooling from 1000C to 00C:

Mass of X deposited = (200 -175) = 25g

Mass of Y deposited = (200 – 83.3) = 116.7

Mass of Z deposited = (200 - 25) = 175g

Hence, Y and X deposited more 100g of the solute.                                               [E]

44.  The law of conservation of mass states that mass of the reactant before is equal to the mass of the product after reaction. The masses of the reactants and products can only be weighed if they are in solid or liquid forms. HCl and NaCO₃ reacts to form a gaseous product (CO₂) thereby making them unsuitable for the experiment 2NHCl + Na₂CO₃ 2NaCl + H₂O + CO₂                      [E]

45.  In titration between an acid and a base, the base is measured out from a beaker using a pipette and then poured into a conical flask before titrating against the acid. [B]

46.  The burning yellow phosphorus used up the oxygen present in the gas jar thereby decreasing the pressure in the gas jar. The burning phosphorous increases the temperature of the water in a beaker and its vapour pressure. Thus, the greater vapour pressure of water causes the water to rise in the gas jar with a lower pressure      [C]

47.  Any test that identifies a gas completely is a confirmatory test for that gas. The confirmatory tests for the gases include:

Gases	Confirmatory Tests
SO2	Decolourizing of KMnO4 or K2Cr2O7 without a yellow deposit of sulphur
CO2	Turning lime water milky
H2	Burning with lighted splinter to produce a pop sound
CO	Burning with lighted splinter to produce CO2 which turn lime water milky.
NO	Formation of dark-brown solution with acidified FeSO4 solution

From the above table, only CO₂ can be identified completely because the ability to turn lime water milky is a confirmatory test for CO₂                                                    [A]

48.  Titration between a strong acid and a weak base or trioxocarbonates requires methyl orange as an indicator. Other titrations and their respective indicators are:

Titrations	Indicators
Strong acid + strong base	Any indicator
Weak acid + Strong base	Phenolphthalein
Weak acid + Weak base	No suitable indicator

[C]

49.  A basic hydride is a hydride that dissolves in water to form a base. Hydrides of alkali metals, alkali-earth metals and nitrogen dissolve in water to form base. Thus in the hydride -R.H. R is an alkali metal (A)

50.  Atom with the highest ionization energy is most reluctant to lose its electron since atomic size is inversely related to ionization energy, R being the smallest atom will have the highest ionization energy and thus will lose its electron most reluctantly [A]