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WAEC Chemistry questions and answers 2020
Waec chemistry answers 2020 to waec chemistry questions 2020 for Essay and Objectives paper. The waec chemistry question and answer 2020 obj with Essay for West African Examination Council (WAEC) Chemistry SSCE exam paper scheduled to be written on Wednesday, 2nd September, 2020.
The Waec chemistry 2020 Essay paper will start by 2:00 pm and will last for 2hrs while the WAEC chemistry Objective exam will commence 4:00 pm and will last for 1hr.
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WAEC Chemistry Answers 2020:
PAPER 2 [Essay]
Answer any FOUR questions.
Write your answers on the answer booklet provided.
1. (a) (i) What is the structure of the atom as proposed by Rutherford?
(ii) Distinguish between the atomic number and the mass number of an element.
(iii) Explain briefly why the relative atomic mass of chlorine is not a whole number.
(b) (i) What is meant by first ionization energy?
(ii) List three properties of electrovalent compounds
(iii) Consider the following pairs of elements:
9F and 17CL;
12Mg and 20Ca.
Explain briefly why the elements in each pair have similar chemical properties.
(c) Explain briefly the following terms using an appropriate example in each case
(i) homologous series;
(ii) heterolytic fission.
(d) State the indicator(s) which could be used to determine the end-point of the following titrations:
(i) dilute hydrochloric acid against sodium hydroxide solution;
(ii) dilute hydrochloric acid against ammonium hydroxide solution;
(iii) ethanoic acid against sodium hydroxide solution.
(e) A solid chloride E which sublimed on heating reacted with an alkali F to give a choking gas G. G turned moist red litmus paper blue. Identify E,F and G.
ANS: (a) (i) The atom has small/ tiny positively charged centre /nucleus with electrons surrounding the space around the centre.
(ii) Atomic number of an element is the number of protons/electrons in an atom of the element while mass number is the sum of the protons and neutrons in the atom of the element.
(iii) Chlorine atom is made up of a mixture of isotopes and the relative atomic mass of chlorine is the average of its isotopic masses.
(b) (i) Is the (minimum) energy required to remove one mole of an electron from one mole of gaseous atom (to form one mole gaseous charged ion
(ii) High melting /boiling point;
Ability to conduct electricity in the molten state or in solution;
Solid at room temperature;
Soluble in water or polar solvents /insoluble in non-polar solvents.
(iii) Atoms of the elements in each pair have the same number of electrons in their outer-most shell therefore similar chemical properties.
(c) (i) Is a family of organic compounds:
– where successive members differ by –CH2 of molar mass of 14;
– with similar chemical properties;
– which conform to the same general formula;
– which show gradation of physical properties;
– which have the same general method of preparation. e.g alkanes, alkenes , alkanols.
(ii) Is a process in which a (covalent) bond is broken in such a way that the electron pair is completely transferred to one of the atoms (resulting in the formation of ions)
H ÷ CI → H+ + Cl-/ HCl ® H+ + Cl-
(d)(i) Methyl orange/ methyl red/ phenolphthalein;
(ii) Methyl orange/ methyl red;
(e) E – NH4Cl
F – NaOH, KOH, or Ca (OH)2, Li OH, CsOH, Ba(oH)2, Mg(OH)2
G – NH3.
2. (a) (i) Define saturated solution.
(ii) The solubility of KN03 at 20°C was 3.00 mol dm-3 If 67.0g of KN03 was added to 250cm3 of water and stirred at 20°C, determine whether the solution formed was saturated or not at that temperature.
(b) (i) Distinguish between dative bond and covalent bond.
(ii) Explain why sugar and common salt do not conduct electricity in the solid state.
(iii) State the type of intermolecular forces present in
I. hydrogen fluoride;
(iv) Consider the compounds with the following structures:
S – H —-N and 0 – H —–N
In which of the compounds is the hydrogen bond stronger? Give reason for your answer.
(c) (i) State Dalton’s Law of Partial Pressure.
(ii) If 200cm3 of carbon(IV) oxide were collected over water at 18°C and 700 mmHg, determine the volume of the dry gas at s.t.p.[ standard vapour pressure of water at 18°C = 15 mmHg]
ANS: (a) (i) Is a solution which contains the maximum amount of solute it can dissolve at a given temperature (in the presence of undissolved solute).
(ii) Solubility of KN03 in in g dm-3 = 3.00 x 101 = 303
.. 1000cm3 of saturated solution = 303g
250cm3 of the solution = 303 x 250
= 75.8 g
Since the quantity of KN03 added (67.0) to 250 cnr’ of water is less than maximum amount required to form a saturated solution, then the solution is unsaturated.
(b)(i) In dative bond, only one of the participating atoms/ species donated electrons to be shared by both atoms while in covalent bond both participating atoms/ species contribute equally the electrons being shared.
(ii) Sugar is covalent while common salt (NaCl) is electrovalent/ ionic. Electrical conductivity (in compounds) depends on presence of mobile ions.
(iii) The intermolecular forces present in hydrogen fluoride and argon were hydrogen bond and van der Waal’s forces respectively.
(iv) 0 – H —- N has the stronger hydrogen bond because oxygen is more electronegative and smaller in size than sulphur.
(c)(i) the total pressure exerted by a mixture of gases which do not react chemically is equal to the sum of the individual partial pressures of the gases in the mixture.
(ii) pressure of the dry gas (P 1) = 700 – 15 = 685 mmHg
VI = 200cm3, TI = 18°C = 273 + 18 = 291K, P2 = 760 mmHg,
T2 = 273
P1V1 = P2V2
V2 = P1V1T2 =
= 685 x 200 x 273
760 x 291
3. (a) (i) Define nuclear fission
(ii) A certain natural decay series starts with and ends with. Each step involves the loss of an alpha or a beta particle. Using the given information, deduce how many alpha and beta particles were emitted.
(b) Consider the equilibrium reaction represented by the following equation: A2(g) + 3B2(g) 2AB3(g); H = + kJmol-1
Explain briefly the effect of each of the following changes on the equilibrium composition;
(i) increase in concentration of B;
(ii) decrease in pressure of the system;
(iii) addition of catalyst.
(c) The lattice energies of three sodium halides are as follows:
Explain briefly the trend.
(d) State the property exhibited by nitrogen (IV) oxide in each of the following reactions:
(i) 4Cu + 2NO2 4CuO + N2;
(ii) H2O+ 2NO2 HNO3 + HNO2
(e) Iron is manufactured in a blast furnace using iron ore (Fe2O3), coke and limestone.
Write the equation for the reaction(s) at the:
(i) top of the furnace;
(ii) middle of the furnace;
(iii) bottom of the furnace.
(f) (i) Name two products of destructive distillation of coal.
(ii) Give one use of each product in 3(f)(i).
4. Copy and complete the following table:
|Oxide of nitrogen||Oxidation state of nitrogen||Colour of gas||Solubility in water||Action on damp blue litmus paper|
(i) Mention one compound that makes water
I. temporarily hard;
II. permanently hard.
(ii) State one method that could be used to remove
I. only temporary hardness;
II. permanent hardness.
(iii) Write an equation to show the removal of:
I. temporary hardness;
II. permanent hardness.
(c) (i) List three sources of water pollution.
(ii) Mention two ways by which water pollution can be controlled.
(d) state the function of each of the following substances in the purification of water for town supply:
I. sodium aluminate (III) (NaAIO2);
II. lime (calcium hydroxide);
III. calculated mount of iodine;
IV. sand bed.
6. (a) (i) What is meant by atomicity?
(ii) Mention one element in each case which is
(iii) Write the orbital electron configuration of
1. In which group does each of the elements belong?
2. How many unpaired electrons are present in 9F?
3. How many electrons are present in 20Ca2+?
(b) (i) Write a balanced equation for the thermal decomposition of KCƖO3.
(ii) Mention the catalyst that could be used to increase the rate of reaction in 6(b)(i).
(ii) If 5.0 g of KCƖO3 was decomposed by heat, determine the volume of oxygen produced at s.t.p.
[Molar gas volume at s.t.p. = 22.4dm3, K = 39, Cl = 35.5, O = 16]
(c) (i) Mention the products formed when each of the following substances is heated strongly:
(ii) State the colour change observed when each of the residues in 1(d)(i) above is allowed to cool.
7. (a) Describe briefly how each of the following aqueous solutions could be identified in the laboratory:
(i) Ammonium trioxocarbonate (IV);
(b) Arrange the following compounds in order of increasing boiling point and give reasons for your answer: CS2, NaF and CO2.
(c) List two gases each that are:
(ii) highly soluble in water;
(iii) oxidized by acidified KMnO4(aq).
(d) In a tabular form, compare the elements silicon and sulphur under the following properties:
(i) metallic character;
(ii) physical state;
(iii) conduction of electricity.
(e) A cuboid piece of sodium metal measures 3 cm x 4 cm x 10 cm. If the density of sodium is 0.971 g cm-3, calculate the number of atoms in the sodium metal.
[ Na = 23; Avogadro constant = 6.02 x 1023 mol_1 ]
8. (a) (i) Define standard electrode potential.
(ii) State two factors that affect the value of standard electrode potential.
(iii) Give two uses of the values of standard electrode potential.
(iv) Draw and label a diagram for an electrochemical cell made up of
Cu2+/Cu; = + 0.34
Zn2+/Zn; = – 0.76
(v) Calculate the e.m.f of the cell in 8(a)(iv) above
(b) (i) In terms of electron transfer, define
II. oxidizing agent.
(ii) Balance the following redox reaction: MnO4- + I- H+ I2Mn2+
(c) Classify each of the following oxides as basic, amphoteric, acidic or neutral:
(i) Carbon (II) oxide;
(ii) Sulphur(IV) oxide;
(iii) Aluminium oxide;
(iv) Lithium oxide.
(d) What is hydrogen bonding?
WAEC Chemistry Objective Questions 2020:
PAPER 1 [Essay]
Answer All questions in this section.
Write your answers on the answer sheet provided.
1. Two immiscible liquids with different boiling points can be separated by _____
A. The use of separating funnel
2. A mixture of CaCl2 and CaCO3 in water can be separated by ______
3. What is responsible for metallic bonding?
A. sharing of electrons between the metal atoms
B. attraction between the atomic nuclei and the cloud of electrons
C. Transfer of electrons from one atom to another
D. attraction between positive and negative ions.
4. 25cm3 of 1.5M solution of NaCl are added to 50cm3 of 3M NaCl. The molar concentration of the resulting solution is ________
A. 2.5M B. 3M C. 2.25M D. 4.5M
5. A solution of salt formed from HCl and NH3 solutions is _____
A. Acidic B. Basic C. complex D. Neutral
6. Which of the following elements will burn in excess oxygen to form a product that is neutral to litmus? A. carbon B. Hydrogen C. Sulphur D. Sodium
7. A current was passed for 10mins and 0.2mole of Cu was deposited. How many grammes of Ag will it deposit?
(Cu = 64, Ag = 108)
A. 43.2g B. 21.6g C. 10.8g D. 5.4g
8. Pollution of underground water by metal ions is very likely in a soil that has high ________
A. Acidity B. Alkalinity C. Chloride content D. Nitrate content
9. Producer gas is a gas with low caloric value because it contains more ____
A. CO2 than O2 B. N2 than CO C. CO2 than N2 D. N2 than CO2
10. Silver chloride turns grey when exposed to sunlight because _____
A. The silver ion is reduced to silver
B. The silver ion is oxidized to silver
C. Silver is a transition metal
D. The silver chloride forms complexes in the sun.
11. Which of these compounds exhibits resonance?
A. Benzene B. Ethanol C. Propene D. Butyne
15. Hydrolysis of CH3COOCH2CH3 in dilute HCl produces ______
A. CH3COOH + CH3CH3
B. CH3CH2OH + CH3COCl
C. CH3COOH + CH3CH2OH
D. CH3COOH + CH3CH3
16. Calculate the volume of CO2 measured at s.t.p produced on heating 250g of potassium hydrogen trioxocarbonate (IV) strongly.
(K = 39, H = 1, C = 12, O = 16)
A. 28dm3 B. 2.8dm3 C.5.6dm3 D. 11.2dm3
17. The boiling points of water, ethanol, methylbenzene and butan-2-ol are 373.0K, 351.3K, 383.6K and 372.5K respectively. Which liquid has the highest vapour pressure at 323.0K?
A. Water B. Methylbenzene C. Ethanol D. Butan-2-ol
18. The conclusion from Rutherford’s alpha scattering experiment is that ______
A. Atoms are mostly empty space with a small nucleus
B. Emissions from radioactive substances consist of three main components
C. There is a nuclear pull on orbital electrons
D. Electrons are deflected by both magnetic and electric fields.
19. Elements P, Q and R have atomic numbers 9, 16 and 20 respectively. Which of them would gain electron(s) during ionic bonding A. Q and R
B. P and R
C. P and Q
D. P, Q and R.
20. Which of the following has the lowest PH?
A. 5cm3 of M/10 HCl
B. 10cm3 of M/10 HCl
C. 20cm3 of M/8 HCl
D. 15cm3 of M/2 HCl
21. Which of the following is an acid salt?
A. (NH4)2CO3 B. CHCOONa C. KHSO4 D. MgSO4.7H2O
22. Cr2O2-7 + 14H+ + 6I → 2Cr3+ + 3I2 + 7H2O. The change in the oxidation number of oxygen in the equation above is _______
A. 0 B. 1 C. 2 D. 7
23. During electrolysis of CuSO4 solution using platinum electrodes, which of the following occurs ______
A. Acidity increases at the cathode
B. Oxygen is liberated at the cathode
C. PH decreases at the cathode
D. PH of solution increases.
24. Which of the following ions is a pollutant in drinking water even in trace quantities?
A. Ca2+ B. Pb2+ C. Mg2+ D. Fe2+
25. The solubility of a salt of molar mass 100g at 20oC is 0.34mol/dm3. If 3.4g of that salt dissolved completely 250cm3 of water at that temperature, the resulting solution is ________
A. A suspension
26. Catalyst is important in in chemical industry in that ______
A. it affects the purity of the products
B. it affects the quantity of the products
C. it increases the time for reaching equilibrium
D. Bond breaking is slowed down.
27. An alkanioc acid has a molecular mass of 88. Name the acid.
(C = 12, O =16, H = 1)
A. Propanioc acid
B. Botanioc acid
C. Pentanioc acid
D. But-2-ionic acid
28. Ethyne undergoes the following reactions EXCEPT
A. Polymerization B. Addition C. Combustion D. Etherification