1984 JAMB CHEMISTRY ANSWERS

1984 CHEMISTRY ANSWERS

1.     Brine is a concentrated solution of sodium chloride. To obtain the solid NaCl from it, the brine will be subjected to heat to vapourize the water leaving behind a solid residue of NaCl [D]

2.     Equation for the reaction:

2H2 + O2 2H2O

Applying Gay Lussac’s law:

2 volumes of H2 reacted with 1 volume of O2 to form 2 volumes of H2O.

20cm3 of H2 will react with 10cm3 of O2 to form 20cm3 of steam.

The cooling of the mixture turns the steam to water which is absorbed by CaCl2. The residual gas is therefore the unreacted oxygen.

Volume of unreacted oxygen = (20 -10) = 10cm3 [D]

3.     From the equation, 1 mole (64g) of NH4NO2 produced 1 mole of N2.

3.2g of NH4NO2 will produce:

    ×    = 0.05 mole of N2

1 mole of N2 at s.t.p = 22.4dm3

0.05 mole of N2 = 1.12dm3 [D]

4.     In the laboratory preparation of Cl from HCl using MnO2 as oxidizing agent, the equation for the reaction is: MnO2 + 4HCl MnCl2 + 2H2O + Cl2

Comparing this with the equation below:

MnO2 + xHCl MnCl2 +yH2O + Cl2

Therefore, x = 4, y = 2 [D]

5.     A molar solution of of a solute contains 1 mole of that solute in 1000cm3 of the solvent.

Mass of 1 mole of NaOH = 40g

Thus, a molar solution of NaOH will contain 40g of NaOH in 1000cm3 of water.

NB: 100cm3 of water is the same as 1000g of water because the density of water = 1gcm-3[B]

6.     FF

7.     According to Boyle’s law, the volume of a given mass of an ideal gas at constant temperature is inversely proportional to its pressure.

Thus, a graph of volume (V) against pressure (P) gives a downward slope from left to right while a graph of V against 1/P will give an upward slope from left to right as shown by graph 3 [C]

8.     Naphthalene is a molecular solid whose molecules are held together bu the weak Van der Waals forces which is broken at a temperature of 810C rendering the solid more mobile [E]

9.     Hydrogen and oxygen are diatomic gases having two atoms in one molecule of the element.

One molecule of hydrogen and oxygen are written as H2 and O2 respectively.

Mass of 1 molecule of H2 = (2 × 1) = 2g

Mass of 1 molecule O2 = (2×16) = 32g

Thus, 2g of H2 contain 1 molecule

16g of O2 contains   ×    = 0.5 molecule

Ratio of H2 to O2 molecule = 1 : 0.5 = 2:1 [A]

10.  Rates of chemical reaction can be increased by the following ways: addition of catalyst to lower the activation energy – (1), increasing the temperature to increase the average kinetic energy of the reactant particles – (3), powdering the solid reactants to increase the surface area for the reaction – (5)  [B]

11.  Aluminium hydroxide also known as mordant is amphoteric, thus it can dissolve in both acid and alkali to form different salts. It reacts with H2SO4 to form Al2(SO4)3 shown in the equation below:

3H2SO4 + 2Al(OH)3  6H2O + Al2(SO4)3 [D]

12.  When a saturated solution is cooled below the saturation temperature, the solute begins to crystallize out of the solution. Only the saturated temperature of R and S are above 323K, thus cooling the solution to 323K will cause the solutes in R and S to crystallize [D]

13.  In neutralization reaction, 1 mole of dibasic acid eg H2SO4 reacts with 2 moles of monobasic base eg NaOH and NaHCO3 while 1 mole of dibasic acid reacts with 1 mole of di-acidic base eg Na2CO3 to form a neutral salt. In option A, the expected mole ratio of the dibasic acid (H2SO4)and di-acidic base (Na2CO3) is 1 : 1

But the mole ratio is (MaVa) : (MbVb) = (0.05×25) : (0.5×25) = 1.25 : 12.5  = 1 : 10. Thus, the base is in excess resulting in a pH greater than 7 [A]

14.  Hydrogen peroxide is an oxidizing agent but in the presence of a stronger oxidizing agent like concentrated HNO3, it acts as a reducing agent and itself transformed into an oxygen gas ie changing the oxidation state of oxygen from -1 in H2O2 to zero in O2 [D]

15.  Halides like iodides react with iron (iii) ion in the equation reducing it to iron (ii) ion and itself being oxidized to iodine [C]

16.  From the graph:

Heat of the product H_P = 40KJ

Heat of the reactant H_R = 60KJ

Enthalpy change H = H_P – H_R

H = 40 -60 = -20KJ

When H is negative, the reaction is said to be exothermic [D]

17.  In the equation, NaOH + HCl   NaCl + H2O; 1 mole of H+ reacts with 1 mole of OH- to form 1 mole of H2O. The heat change is called standard heat of neutralization and this is -57.3KJ. In the equation, 2NaOH + H2SO4  Na2SO4 + 2H2O, 2 moles of H+ reacts with 2 moles of OH_ to form 2 moles of H2O. Enthalpy change = (2 × -57.3) = -114.7KJ [D]

18.  The double arrows in the equations demonstrate a chemical system in equilibrium and every chemical system in equilibrium obeys Le Chatelier’s principle

19.  In the laboratory, concentrated H2SO4 can be used to displace HNO3 from its salt eg NaNO3. The equation for the reaction is

20.  NaNO3 + H2SO4  NaHSO4 + HNO3 [C]

21.  Lime water is formed by addition of water to quicklime. The process is called slaking and the equation for the reaction is as follows

CaO + H2O   Ca(OH)2

22.  Allotropy is a phenomenon whereby an element exists in two or more structural forms in the same physical states

23.  CO is NOT readily oxidized a room temperature but requires an intense supply of heat to react with oxygen to form CO2 [B]

24.  In the first equation, Na2O is a basic oxide. In the second equation, CO2 is an acidic oxide. The ability of zinc oxide to react with acid and base to form salt shows that zinc oxide is amphoteric [D]

25.  A neutral oxide is an oxide that can neither form an acid nor a base when dissolved in water. Such oxides have no effect on moist litmus paper eg CO, N2O and NO [D]

26.  The reaction between chlorine and ammonia is a redox redox reaction whereby the ammonia is oxidized by chlorine to nitrogen gas and the chlorine is in turn reduced to hydrochloric acid by the ammonia. Thus, the ammonia acts as a reducing agent [A]

27.  In the industrial preparation of ammonia (Haber process), nitrogen reacts reversibly with hydrogen in the presence of  finely divided iron as a catalyst at a temperature of 4500C and pressure of 200 atm to yield about 15% of ammonia [C]

28.   

	C	H	O	N
% comp.	53.1	6.2	28.3	12.4
Proportion	= 4.4	= 6.2	= 1.8	= 0.89
Ratio	= 5	= 7	= 2	= 1

Empirical formula = C₅H₇O₂N

Molecular mass = 2 × vapour density = 2 56.5) = 113g

Molecular formula = (C₅H₇O₂N)n

Mass of the empirical formula = 5(12) + 7(1) + 2(16) + 14 = 113g

113n = 113

n =   = 1

Hence, molecular formula is the same as empirical formula = C₅H₇O₂N [D]

29.  Structural formula of ehyne is: H – C  C – H

Triple bond corresponds to SP hybridization. Alternatively, one carbon atom is bonded to one hydrogen atom ie C¹H¹ which corresponds to S¹P¹ = SP [D]

30.  Large molecular hydrocarbons such as diesel, kerosene, etc are broken down to smaller molecular hydrocarbon of lower boiling point like petrol by heating the former at a high temperature. This process is called cracking. It helps to increase the supply of petrol which is the most demanded fraction of petroleum [D]

31.  He functional group- COOH indicates the homologous series – alkanoic acid named as (anoic acid). The longes carbon chain has 3 C-atoms named as (prop). Combining the names in the bracket in a reverse order gives the IUPAC name:  propanoic acid [E]

32.  Alkaline hydrolysis of fats and oil is called saponification reaction. Fats and oil are high molecular esters that react with caustic soda to form sodium octadecanoate (soap) and propane 1,2,3- triol (glycerol)  [B]

33.  Carboxylic acid also known as alkanoic acids are recognized by the presence of the functional group – COOH. They have the general formula RCOOH  [A]

34.  Calcium carbide does NOT form any alkyne on reaction with water. It only forms ethyne. The equation for the reaction is :

CaC₂ + 2H₂O   C₂H₂  + Ca(OH)₂

35.  All the four acids have a H of less than 7, turn blue litmus red, neutralize alkalis to form salts, react with magnesium to liberate hydrogen. However H₂CO₃ does NOT dissolve marble to liberate CO2 [A]

36.  1g of Mg =  mole of Mg

If the liberation of   mole of Mg with a charge of +2 cost #5.00.

   mole of Al with a charge of +3 will cost more

Cost of depositing   mole of Al =   ×   = #7.5

10g of Al =   mole.

If  mole of Al cost #7.5

 mole will cost (  ×  ×   ) =  #66.67 [D]

37.  Electrolytic equation for copper

Cu + 2e-  Cu

From the above equation,

1 mole (63.5g )

Of Cu requires 2 Faradays of electricity to be deposited.

2F = 193000C = 63.5g of Cu

16000C =  ×  = 5.3g = [N.C.O]

38.  From the illustrated atomic number and mass number of the elements, it can be deduced that:

R is  Lithium, U is Fluorine S is Magnesium T is Neon Y is an isotope of Fluorine. Thus, it is wrong to say that R, U, S &T are metals because U and T are non metals [C]

39.  G5

40.  Hard water contains dissolved Ca and Mg ions in the form of HCO₃^- and SO₄^2-. During washing, these ions react with soap to form insoluble scum  (calcium octadecanoate & magnesium octadecanoate) [C]

41.  When sodium hydroxide pellets are exposed into the atmosphere, they absorb a lot water to form a solution. Thus, they are said to be deliquescent. Other deliquescent substances include KOH, CaCl2, MgCl2, FeCl3 P4O10, etc

42.  Isomers have the same number of atoms of the same elements ie the same molecular formula but different structural formula. Options A, B ,C & E have the molecular formula – C4H10O while option D has the molecular formula – C4H8O [D]

43.  Alkanes are hydrocarbons consisting of only carbon and hydrogen. They are relatively unreactive, insoluble in water and have the general formula- CnH2n +2. They can exist in gaseous, liquid and solid states depending on the molecular mass [C]

44.  The first member of alkane (methane) reacts with chlorine in the presence of sunlight as a catalyst to produce a chlorinated hydrocarbon and hydrogen chloride gas. The final product formed is terachloromethane and the reaction is called substitution reaction because atoms of H are replaced  (substituted) by the same atoms of Cl [D]

45.  Esterification reaction between ethanoic acid and ethanol occurs very slowly. In oorder to increase the rate of the reaction, concentrated H2SO4 is usually added as a catalyst [C]

46.  One characteristic feature of trioxocarbonates is the ability to react with dilute acid eg HCl to liberate CO2 which is a colourless and odourless gas that turns lime water milky. Thus, the sea shell must have contained CaCO3 [C]

47.  In the electrolysis of purified bauxite (alumina), the cathode is graphite lining while the anode is graphite rod which is an allotrope of carbon. Oxygen produced at the anode ignites the carbon to form CO2 thereby consuming the anode which has to be replaced. [B]

48.  The salts of zinc, lead and aluminium produce a white precipitate with drops of aqueous ammonia while the salts of lead and aluminium does not. In addition, zinc forms a white precipitate with drops of NaOH which dissolves in excess NaOH [A]

49.  The presence of a single bond indicates the functional group –alkane named as (ane). The longest carbon chain contains 4 C- atoms named as (but). The branch chain is a methyl group at C-2 atome named as (2 methyl). Combining the names in a reverse order gives 2-methyl butane [D]

50.  Equation for the reaction:

Zn + H2SO4  ZnSO4 + H2

From the above equation 1mole of H2SO4produced 1 mole (22.4dm3)  of H2

0.5M H2SO4 means 0.5mole of H2SO4 in 1000cm3 of the solvent

500cm3 of the solvent will contain       = 0.25 . 0.25 mole of H2SO4 will produce 0.25 mole of H2

Volume of H2 = 0.25 × 22.4 = 5.6dm3 [D]