Chemistry 2019 Past Questions | WAEC

Study the following Chemistry past questions and answers for JAMBWAEC  NECO and Post-JAMB. Get prepared with official past questions and answers for upcoming examinations.

46. The number of electrons in the 3d orbital of  24Cr is

  • A. 2
  • B. 3
  • C. 4
  • D. 5

47.  The method used to collect gas in the laboratory depends on its?

  1. boiling point  II. density  III. smell  IV. solubility in water
  • A. i and ii only
  • B. ii and iv only
  • C. iii and iv
  • D. i, ii, iii and iv

48.  Which of the following methods is NOT used  for the separation of mixtures?

  • A. Chromatography
  • B. Crystallization
  • C. Distillation
  • D. Electrolysis

49.  The group to which elements belong in the periodic table is determined by the number?

  • A. electrons
  • B. core electrons
  • C. valence shells
  • D. valence electrons

50. Kipps apparatus can be used to prepare?

  • A. H2
  • B. NH3
  • C. O2
  • D. H2S

CHEMISTRY THEORY PAST QUESTIONS 2019 WITH SOLUTION

1(a) Explain briefly why He has a stable electron configuration compared to Be 
(b) Consider the following elements: 1H and 3Li.
(i)  State the number of electrons that an atom of each element would have after forming an ionic bond.
(ii)  Give a reason for each of your answers stated in (b)(i).               
(c)  State two factors that should be considered when siting a chemical industry.

(d) State two advantages of using a catalyst instead of high temperatures in chemical reactions. 
(e)     Turpentine burns in chlorine according to the following equation:
C10H16(I)    +    8Cl2(g) →    10C(s)   +    16HCl(g)
Calculate the mass of turpentine that would completely burn in 21.3 g of chlorine.
[Molar mass of chlorine = 71 gmol−1; Molar mass of Turpentine = 136 gmol−1]

(f)  What is cracking?                                                                                      


(g) State two factors that may influence the value of electron affinity.


(h) What are carbohydrates?                                                                          


(i) State two differences between a simple sugar and starch.                     

 

(j) Write an equation to show the dissociation of each of the following acids:
(i) H22CO3;
(ii) CH3COOH.

2(a) (i)  Name three different methods for preparing salts.
     (ii) Give one example of a balanced equation for each of the methods named in (a)(i).
     (iii) State two uses of sodium trioxocarbonate (IV). 

(b) If you were given some impure copper, describe how you would
obtain a specimen of the pure metal by electrolysis. 

 

(c) Given that sodium chloride has a solubility of 36.3 at 30  and 39.0 at
100  and that of silver nitrate is 297.0 at 30  and 952.0 at 100.

 

(i) Calculate the percentage of each substance in the saturated solution at 100  that is deposited on cooling to 30


(ii) Deduce which of the two salts can be purified more efficiently by crystallization

3(a) (i) State the important points put forward in Dalton’s atomic theory.
     (ii) How does the theory explain the law of multiple proportion?

 

(b) (i) Name three commercially useful products that may be obtained by chemical transformation of vegetable oils.


(ii) With the aid of equations, describe the chemical reaction involved in the transformations stated in (b)(i). 

 

(c) Certain properties of Beryllium (Be) and its compounds differ from those of Magnesium (Mg) and its compounds, but rather resembles those of Aluminium and its compounds. Explain briefly why this is so.

 

(d) Write balanced equation for the reaction between iodine and aqueous sodium trioxothiosulphate(VI).  

4(a)  (i)  What is meant by hardness of water?
      (ii) Describe briefly how you would determine what proportion of hardness in a given sample of water is due to permanent hardness.
     (iii) Give two reasons why hardness of water is an undesirable property.

(b)   State the:
(i)    reagents;
(ii)   conditions for the laboratory preparation of trioxonitrate (V) acid.
(iii)  How does concentrated trioxonitrate (V) acid reacts with:
(I)   sulphur;
(II)  aluminium.  

(c)   Name one amphoteric oxide.

5(a) (i) Write an equation for the reaction by which sulphur dioxide in solution could be converted to tetraoxosulphate (VI) acid.

     (ii) State one test to confirm the conversion of sulphur dioxide to tetraoxosulphate (VI) acid.

     (iii) State the reaction of concentrated tetraoxosulphate (VI) acid with:

(I)  oxalic acid;
(II)  copper.

     (iv) What property of concentrated tetraoxosulphate (VI) acid does each of the reactions stated in  (a)(iii) illustrate? 

(b) (i)  Explain briefly why water is referred to as a universal solvent.
(ii)  Give one chemical test for water.  

(c)  (i)  What is the major component of synthetic gas?
      (ii) Give one reason why synthetic gas is not a major source of air pollution.        

(d) Name one product of destructive distillation of coal that is:

(i) solid;
(ii) liquid;
(iii) gas.

(e) (i) Write a balanced chemical equation for the complete combustion of carbon.

(ii) State one:
(I)  physical;
(II) chemical property of the products in  (e)(i). 

(a)  State the conditions necessary for the cracking of long-chain hydrocarbons to produce more gasoline. 

(b)  State two reasons why metallic objects are electroplated 

(c) (i) Explain briefly why calcium oxide cannot be used to dry hydrogen chloride gas.
(ii) State one drying agent for hydrogen chloride gas.   

(d)  Concentrated trioxonitrate (V) acid was added to a solution of iron (II) tetraoxosulphate (VI) and the mixture heated. The mixture turned from pale green to yellow with the evolution of a brown gas. Explain briefly these observations.

(e)  (i) Write the equation for the reaction between zinc oxide and

  1. dilute tetraoxosulphate (VI) acid,
  2. sodium hydroxide solution.(ii)  State which property of zinc oxide is shown by the reaction in (e)(i).

(f)  Two isotopes of chlorine are 35/17Cl and 37/17Cl State one:

  1. similarity;
  2. difference between the isotopes.

(g) State the two products formed when chlorine water is exposed to sunlight.                                 (h) Consider the reaction represented by the following equation:

   State the:

  1. species that is undergoing oxidation;
  2. oxidizing agent.  

(i)  What is meant by carbon-12 scale?  

(j) State two properties of a chemical system in equilibrium.

6(a) A hydrocarbon having the formula C10H2 was cracked to produce C6H14 and another hydrocarbon P.

(i) Give the molecular formula of P.

(ii) Draw the structures of two isomers of P.     

(ii) Give a reason why P could be polymerized.

(b) State the guiding principles which are used to explain the way electrons of the atoms of the elements are arranged in atomic orbitals.

(c)Consider each of the following substances: NaH, H2, H2S, NH4Cl.

(i) Describe the nature of the intermolecular forces holding the units or molecules together in the condensed (liquid or solid) state.

(ii) Explain briefly what happens when a sample of each of the substances is added to water.

 (iii) Write the chemical equations of any reactions occurring or of any equilibria established.    

(d) Element J has the following electron configuration: Is2s2p63s2

(i) How many unpaired electrons can be found in J?

(ii) State whether J would be a good oxidizing or reducing agent.

(iii) Give a reason for the answer in (d)(ii).

8(a) The following reaction scheme is an illustration of the contact process. Study the scheme and answer the questions that follow.

(i) Name X and Y

(ii) Write a balanced chemical equation for each of the processes I, II, III and IV 

(iii) Name the catalyst used in process II

(iv) Using Le Chatelier’s principle, explain briefly why increasing the temperature would not favour the reaction in II

(v) State two uses of SO2  

(b) Consider the following equation: 2H2(g) + O2(g)→ 2H2O(g)
Calculate the volume of unused oxygen gas when 40 cm33 of hydrogen gas is sparked with 30cm33 of oxygen gas

(c) Calcium carbonate of mass 1.0 g was heated until there was no further change.

  • Write an equation for the reaction which took place.
  • Calculate the mass of the residue.
  • Calculate the volume of the gas evolved at s.t.p.
  • What would be the volume of the gas measured at 15  and 760 mm Hg?  [C= 12.0, O = 16.0, Ca = 40.0, molar volume of a gas at s.t.p. = 22.4 dm33 ]

9(a)(i)   Draw and label a diagram to illustrate the preparation and collection of dry chlorine gas in the laboratory.
(ii)  State two uses of chlorine.
 

(b) Describe the preparation of hydrogen from water gas.

(i)   Name the chief ore of aluminium.

(ii)  Why is the ore purified?
(iii)  Name the electrode used in the electrolysis.
(iv)  Give one reason why cryolite, NaAlF6, is added to the electrolyte.
 

(c) Name three products obtained directly from the destructive distillation of coal.                                       

All your burette readings (initials and final) as well as the size of your pipette must be recorded but no account of experimental procedure is required. All calculations must be done in your answer booklet.

 A is a solution containing 15.8 g dm3 of Na2S2O3. B was obtained by dissolving 9.0 g of an impure sample of I2 in aqueous Kl and the solution made up to 1 dm3.

(a) Put A into the burette and titrate it against 20.0 cm3 or 25.0 cm3 portions of B. Use starch solution as indicator. Repeat the titration to obtain concordant titre values. Tabulate your results and calculate the average volume of A used. The equation for the reaction involved in the titration is I2 + 2S2O3→→ 2I−−+ S4O2−62−.

(b) From your results and the information provided, calculate the:

(i) concentration of A in mol dm−3

(ii) concentration of I2 in B in mol dm−3;

(iii) percentage by mass of I2 in the sample

(c) Give reasons why the starch indicator was not added to the titration mixture at the beginning of the titration. [O = 16.0, Na = 23.0, S = 32.0, 1 = 127.0] Credit will be given for strict adherence to the instructions for observations precisely recorded and for accurate inferences. AIl tests, observations and inferences must be clearly entered in the booklet in ink at the time they are made.

10. All your burette readings (initials and final) as well as the size of your pipette must be recorded but no account of experimental procedure is required. All calculations must be done in your answer booklet.

C is a mixture of two inorganic compounds. Carry out the following exercises on C. Record your observations and identify any gas(es) evolved. State the conclusions you draw from the result of each test.

(a) Put all of C in a boiling tube and add about 10 cm33 of distilled water. Shake thoroughly and filter. Keep both the residue and the filtrate.

(b)(i) To about 2 cm3 of the filtrate add a few drops of Silver trioxoitrate (V) followed by dilute HNO3

(ii)  Add excess NH3 solution to the resulting mixture in (b)(i).

(C)(i) Put the residue in a test tube, add about 2 cm3 of dilute HCl and shake.

(ii) Add NH3 Solution in drops to the mixture from (c)(i) and then in excess.

s of CU and Zn have the same number of?

  • A. nucleon
  • B. electrons
  • C. neutrons
  • D. protons

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