1980 JAMB CHEMISTRY ANSWERS

UME 1980 CHEMISTRY ANSWERS

1.     To remove ammonium chloride from a mixture of sand, sugar and ammonium chloride solutions, the water in the mixture should be first  evaporated to obtain dry sand, sugar and ammonium chloride. The ammonium chloride is then removed by sublimation [D]

2.     Duralumin is an alloy of aluminum, copper magnesium & manganese. It is used for constructing aeroplanes, ships, cars and machinery because of its strength and lightness.[D]

3.     When a solution reacts with AgNO₃ solution to form a white precipitate soluble in NH₃. It indicates the presence of Cl^-. Thus, solution X contains chloride ion. Chlorides produce white precipitate which dissolves on boiling and reappears on cooling when they react with lead (II) trioxonitrate (V). Thus, solution Y contains Pb²⁺ [B]

4.     Equation for the reaction 2CO + O₂ 2CO₂

Mole ratio 2:1                      : 2

Volume ratio 10:5    : 10

Vol. of air that reacted = 5cm³

Vol. of unreacted air = (100 – 5) = 95cm³.

 Vol. of CO₂ formed = 10cm³.

Vol. of the resulting gases = vol. of CO₂ formed + vol of unreacted air = (10 + 95) = 105cm³ [C]

5.     The mass of mercury oxide cannot be equal to the mass of the mercury metal.

WRONG QUESTION [E]

6.     Equation for the reaction

 X + O₂   X_aO_b

Vol. ratio 1: 1½  = 2 : 3

Mole ratio 2:3

No of atoms of X that reacts = 2.

No of atoms of O that reacted = (2 x3) = 6

Formula = X₂O₆ divide by 2 = XO₃ [A]

7.     This is dilution reaction

V₁ = 1dm³  

V₂= (1+10) = 11dm³

C₁ = 2.2m³

C₂ = ?

Using dilution formula: C₁V₁ = C₂V₂

2.2 × 1 = 11C₂

C₂ =  = 0.2     [E]

8.     Equation for the reaction:

CaCO3 + 2HCl CaCl2 + H2O + CO2

From the above equation:

1 mole of CaCO3 reacted with 2 moles of HCl

1g of CaCO3 = 0.01mole

0.01 mole of CaCO3 will react with 0.02 mole of HCl

Volume of HCl =  =  = 0.2dm³ = 200cm³

Equation for the 2^nd reaction

Na₂CO₃ + 2HCl NaCl + H₂O + CO₂

Using the volumetric formula:

 =    =

Va =   = 50cm³

Original volume of the acid = V1 + V2

= (200 + 50)   250cm³    [D]

9.     Sodium hydroxide can react with tetraoxosulphate(VI) acid to form an acidic salt & water. The correct balanced equation for the reaction is shown below:

NaOH + H₂SO₄ NaHSO₄ + H₂O   [D]

10.  Equation for the reaction:

Mg + 2HCl MgCl2 + H₂

From the above equation:

24g of Mg liberated 2g of H

6g of Mg will liberate

 ×  = 0.5g of H    [C]

11.  Molecules of real gases at high pressures become less energetic. Thus, they do not obey gas laws    [B]

12.  P₁ = (752 -32) = 720mmHg, V₁ = 500cm³ T₁ = (273 + 30) = 303K, P₂ = 760mmHg, T₂ = 273K

1.             =

2.              =    V2 = 426.8  427cm³ [E]

13.  According to the Kinetic theory of gases, the mass of gas particles are not negligible but the volume or the size occupied by gas molecules or particles is negligible [D]

14.  In the electrochemical series, not all metals are above hydrogen. Metals like Cu²⁺, Hg²⁺, Ag⁺ & Au³⁺ are below H⁺ in electrochemical series [D]

15.  Mass of hydrated salt = 5g

Mass of anhydrous salt = 4.26g

Molecular mass of anhydrous salt = 208g

Molecular mass of hydrated salt = x

 =

X =  = 244g

Mass of water = (244 – 208) = 36g

1      molecule of water = 18g.

No. of molecules in 36g of water =  = 2 [D]

16.  The stronger the intermolecular forces, in a compound, the higher the boiling point. Thus water has a higher boiling point than methanol because of the stronger intermolecular forces in water                             [B]

17.  Chlorine, bromine and lodine are halogens which undergo a common reaction of displacing one another. The more electronegative element tends to displace the less electronegative element. Thus Cl displaces Br. While Br displaces I [C]

18.   

Elements	No. of outer electrons
Aluminium	3
Chlorine	7
Silicon	4
Magnesium	2
Sodium	1

From the above table, 3 which corresponds to Aluminium should be placed between 4 and 2 to give a correct decreasing order of 7,

4, 3, 2, & 1. Thus, Aluminium is wrongly placed  [B]

19.  An element with a total number of 19 electrons is potassium. Potassium oxide when strongly heated does not produce the metallic element.  [E]

20.  All alkali metal salts are soluble. Thus they dissolve in water to dissociate into their component cations and amino which makes them good conductors of electricity [A]

21.  C₂H₄ + Cl2    C₂H₄Cl₂

Cl             Cl

H – C – C – H

       1     1

       H     H

Ethene as an unsaturated compound combines with chlorine gas to form a saturated compound 1,2-dicliloroethane. This is called addition reaction   [B]

22.  H₂SO₄ + 2NaOH→Na₂SO₄ + H₂O

From the above equation 2 moles of NaOH requires 1 mole of H₂SO₄

Since the volume are the same 1 mole of NaOH will require 0.5 mole of H₂SO₄. But the given molarity of H₂SO₄ is 1. This means that theracid there will be unreacted H₂SO₄ making the solution acidic [A]

23.  CH₃COOH + C₂H₅OH → CH₃COOC₂H₃ + H₂O. the reaction of ethanoic acid and ethanol in the above reaction is called esterification whereby the ester ethylecthanoate (CH₃COOC₂H₅) is formed [A]

24.  Air is a mixture of gases namely: Oxygen, nitrogen, carbondioxide, inert gases and water vapour when air passes through alkaline pyrogallol, oxygen and carbondioxide are absorbed. When the remaining gases pass through CaCl2, water vapour is absorbed. The remaining gases are nitrogen and inert gases. [E]

25.  Let the percentage composition of 35Cl be x and 37Cl be 100-x

35x + 37 (100-x) = 35.5 (100)

35x + 3700 – 37x = 3550

-2x = 3550 – 3700

-2x = -150

X =  = 75%

Percentage composition of the  isotope of mass no 35 = 75%

26.  2HCl + FeS FeCl₂ + H₂S

The gas evolved is hydrogen sulphide which

blackens lead acetate paper [C]

27.  The oxidation states of Oxygen in H₂O, OH^-

H₃O⁺, OCl^-, SO₂ are -2 while the oxidation state of oxygen in hydrogen peroxide (H₂O₂) is -1 [D]

N.B: the oxidation states of sulphur in SO₂ is +4

28.  In the test for chlorides silver trioxonitrates (V) reacts with chlorine containing salt to form a white ppt of silver chloride (AgCl) which readily dissolves in aqueous ammonia [C]

29.  During electrolysis, negative ions (anions) migrate towards the anode where they lose electron to be liberated as a neutral atom. Oxidation is the removal of electron [A]

30.  When a zinc rod is introduced into a solution of copper (II) sulphate, the zinc will lose electron and dissolve releasing its heat of ionization. The blue solution changes due to the discharge of the less electropositive copper from the solution [A]

31.  Hardening of oil which is employed in the synthesis of margarine involves addition of H in the presence of a catalyst eg nicked [D]

32.  Aqueous sodium chloride consists of Na⁺ and H⁺ as the cations and Cl^- and OH^- as the anions. During electrolysis the less electropositive H+ will be discharged at the cathode leaving behind Na+. the less concentrated Cl^-  will be discharge at the anode leaving behind the more concentrated OH^-. Thus the liquid left will consist of Na⁺ + OH^- i.e NaOH solution [A]

33.  This is a test for copper (II) ions. When treated with small amount of aqueous ammonia a blue precipitate of Cu(OH)₂ will be found. When excess aqueous ammonia is added, the blue precipitate of Cu(OH)₂ dissolves to form a deep solution of complex ion Cu(NH₃)₄ ²⁺ .(OH)₂ [D]

34.  Presence of double bond at C-2 atom indicates the homologous series alkene named as (-2-ene) The longest carbon chain has 5 C- atoms named as (pent).The branch chains are 2 methyl-groups at C-2 and C-3 named as (2,3-dimethyl). Combining the names in the brackets in the reverse order gives: 2,3-dimethylpent-2-ene [B]

35.  In the test for tetraoxosulphate(VI) barium chloride is added to the unknown salt solution. Acidified with dilute HCl. The formation of white precipitate insoluble in dilute H₂SO₄ confirms the presence of SO₄^2- BaCl +XSO₄ BaSO₄ + XCl [B]

36.  Sulphur dioxide dissolves in water to form trioxosulphate(IV) acid solution

H₂O + SO₂ H₂SO₃. This reacts with chloride to form tetraoxosulphate(VI)acid and hydrochloric acid

H₂SO₃ + H₂O + Cl₂ H₂SO₄ + 2HCl [C]

37.  A molecule of Bromine – Br2 will cleave a single bond to saturate a compound with double bond. 2 molecules of bromine is required to cleave a double bond in order to saturate a compound with triple bond can take up 2 molecules of bromine [B]

38.  In the reaction, sodium dichromate completely oxidizes ethanol to ethanoic acid C₂H₅OH +2(O) CH₃COOH +H₂O [A]

39.  Acids conduct electricity and they are decomposed by electric current into their component ions. Hydrogen ion being a cation would migrate to the cathode (NOT anode) where it is liberated as a hydrogen atom under favourable condition that favours its preferential discharge [C]

40.  Since chlorine is 2.5 times denser than air it is best collected by upward displacement of air. It cannot be collected over or under water because it dissolves in water to form hydrochloric acid and oxochlorated(I)acid. Cl₂ +H₂O HCl +HOCl [B]

41.  Cellulose and starches are polysaccharides also known as complex carbohydrates with long chains of monosaccharide  [D]

42.  Magnesium has a very strong affinity for oxygen. When it burns in air to form Magnesium oxide, it hardly loses the oxygen instead the intense heat produced will further ignite any element that comes in contact with it. [B]

43.  The boiling point of organic compounds increases with increase in the molecular mass and a decrease in the number of branches (branched compounds have weak vander waal forces which are easily broken by heat)  [D].

44.  NO₂ is an acid anhydride which dissolves in water to form an acid with a pH less than 7

2NO₂ + H₂O HNO₂ + HNO₃

45.  Detergents are more advantageous than soaps because they do not react with dissolved Ca, Mg or H ions. Thus they neither hydrolyse in acidic water nor form scum with hard water like soap. Hence on treatment with hard water they form more lather than soap.   [E].

46.  Only unsaturated compounds such as alkenes and alkynes undergo rapid oxidation with bromine water [B]

47.  From the graph, when the curve is steepest, it means a faster decrease in the mass of the reactant and thus increased rate of reaction. This occurred at the initial slope of the graph when small pieces of marble more used due to the larger surface area of reactants [C]

48.  In endothermic reaction heat energy is absorbed from the surrounding and the enthalpy change is positive (+∆H) [D].

49.  Since the enthalpy change of the reaction is negative (-∆H), the forward reaction is exothermic and backward reaction endothermic the backward reaction will be favoured by increased temperature. Removal of oxygen will favour the backward reaction so as to replace the removed O₂. The backward reaction involves an increase in mole and volume. Hence a decreased pressure will favour the backward reaction [C]

50.  In CuSO₄ solution, the cations present are Cu²⁺ and H⁺ and the anions are SO₄^2- and OH^-. The copper anode being in contact with the solution loses electrons, which flow into the circuit to  light the bulb. The dissolved copper migrates towards the cathode where it is deposited.  With time the copper anode becomes completely used up.