WAEC 2020 - CHEMISTRY ANSWER

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CHEMISTRY OBJ
1-10: CACDBDADAD
11-20: AADCDABBCD
21-30: BACBCCBCAC
31-40: CBACBCDBAD
41-50: DBBDCBCCCB
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ANSWERS:


(1ai)
Faraday's first law of electrolysis states that the mass(m) of an element discharged during electrolysis is directly proportional to the quantity of electricity(Q) passing through it.

(1aii)
(i) Strong electrolytes conduct large currents while Weak electrolytes do not conduct current readily.
(ii) Strong electrolytes ionize completely while weak electrolytes ionize only slightly.

(1b)
Ethyne burns in air to give a smoky and luminous flame (complete combustion)

(1ci)
If an organic compound contains carbon atoms joined by double or triple covalent bonds. The compound is said to be an unsaturated hydrocarbon.

1cii)
CH3COOH(aq) + CH3OH(aq) <--> CH3COOCH3(l) + H2O(l)

(1ciii)
Methyl ethanoate

(1d)
By removing the main product continuously

(1e)
Number of moles of Zn dust = Mass/molars mass = 3.75/65 =0.0077
Reacting mole ratio of Zn to H2 is 1:1
:’ No of moles of H2 produced =0.0677 moles
1 mole = 6.02x10²³ molecules
0.0577moles of hydrogen produces
=6.02x10²³ x0.0577
=0.347x10²³ molecules
Or 3.47x10²² molecules

(1f)
Flooding

(1gi)
Reaction C

(1gii)
Zinc (from + 2 to + 4)

(1hi)
(i) Each spectral line is caused by one electron
(ii) Electron can exist only in circular orbit of definite quantum energy.

(1hii)
It could not account for the spectrum at note complicated atoms

(1i)
(i)Temperature of the reacting system
(ii)Pressure of the reacting system
(iii)Concentration of the reacting system

(1j)
(i) Both liberate carbon(iv) oxide
(ii) One produce a chloride salt , the other produce an ethanoate salt




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(2ai)
They are isomers

(2aii)
A - Trans 1,2 dichloroethane

B - Cis 1,2 dichloroethane

(2aiii)
Yes

(2aiv)
They contain the same functional group the single bond. The only difference is the polarity(physical property)
The Cis - polar compound while Trans-1, -di chloroethane is non polar

(2bi)
(i) Temperature remains constant during boiling
(ii) There is change of state during boiling

(2bii)
I - It will increase the boiling point of water.
II - It will reduce the boiling point of water.

(2biii)
Boiling occurs at a specific temperature While Evaporation occurs at almost all temperatures.

(2ci)
An unsaturated solution is one that contains less than the maximum amount of solute that can dissolve at a given temperature While A saturated solution is one that contains maximum of solute that can be dissolved in it at that temperature.

(2cii)
(i) By increasing the temperature of the solution.
(ii) By increasing the amount of the solute.

(2ciii)
Temperature.

(2di)
Mole is a standard scientific unit for measuring large quantities of very small entities such as atoms, molecules, or other specified particles

(2dii)
MgO(s)+2HCl(aq) ---> MgCl(aq)+H2O(l)
From the equation of reaction; 1 mole of MgO = 2 moles of HCl
Hence n = cv/100
where c = 0.1moldm-³; v = 25.0cm³
No of mole of HCl = n = 0.1x25/1000
= 2.5 x 10-³ moles
Hence the no of moles ofMgO required = 2.5/2 x10-³
But n = m/M = reacting mass/ Molar mass ---> m = nM = 1.25x10-³ x 40
= 50 x 10-³ = 0.05g of MgO.

(2e)
(i) They are malleable
(ii) They are ductile
(iii) They are good conductors of heat and electricity


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(3ai)
Draw the diagram

(3aii)
I. Activation Energy = (210 - 100)KJ = 110K
II. Hp - HR = ΔH
54 - 100 = -46KJ

(3aiii)
Q2(g) + R2(g) --> 2QR(g)

(3aiv)
Exothermic reaction.

(3av)
The heat content of the reactant is higher than that of the product.

(3bi)
E7 - 1s² 2S² 2P³
F9 - 1s² 2S² 2P⁵
G12 - 1S² 2S² 2P⁶ 3S²
H13 - 1S² 2s² 2P⁶ 3S² 3P¹

(3bii)
I. G and H
II. H
III. F
IV. GF2

(3ci)
Isotopy is the existence of an element in more than one physical form with the same atomic number but different atomic masses.

(3d)
(i) Atoms
(ii) Molecules
(iii) ions


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(4ai)
I
(i) Potassium tetraoxomanganate (vii)
(ii) Hydrochloric acid

II - Concentrated tetraoxosulphate (vi) acid
III - Downward delivery

(4aii)
2KMnO4(aq) + 16HCl(aq) ---> 2MnCl2(aq) + 2KCl(aq) + 8H2O(l) + 5cl2(g)

(4aiii)
3Cl2(g) + 6NnOH(aq) ----> NaClO3(g) + 5NaCl(aq) + 3H2O(l)

(4bi)
(i) Iron (III) oxide
(ii) Coke
(iii) Limestone

(4bii)
C(s) + O2(g) ----> CO2(g)
CO2(g) + C(s) ----> 2CO(g)
FeO3(s) + 3CO(g) ----> 2Fe(l) + 3CO2(g)
CaCO3(s) ---> CaO(s) + CO2(g)
SiO2(g) + CaO(s) ----> CaSiO2(s)

(4biii)
Pig iron

(4biv)
The iron has a relatively low melting point because it contains impurities

(4ci)
It is not dissolved directly in water because the heat evolved during the process would cause the acid solution to boil, producing a mist of acid droplets which would spread throughout the factory

(4cii)
It is regarded as a heavy chemical because it is produced in very large quantities and used in many chemical processes

(4ciii)
I. Displacement reaction
II, As an oxidizing agent

(4d)
2C3H7OH(aq) + 2Na(s) ----> 2C3H7ONa(aq) + H2(g)



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(5ai)
(i) Ammonia (NH3)
(ii) Hydrogen chloride gas(HCl)

(5aii)
To show that hydrogen chloride is extremely soluble in water.

(5aiii)
Fill a round-bottomed flask with dry hydrogen chloride through a tube A and air through another tube B. Add a few drops of water through B and close the tubes. Invert the flask into a trough of water containing blue litmus solution. The water turns red.

(5bi)
Petrochemical industries.
Pharmaceutical industries.

(5bii)
(i) Pollution of water bodies by chemical waste.
(ii) Pollution of air.
(iii) Radioactive particles are released.

(5ci)
(i) Coke.
(ii) Coal tar.
(iii) Coal gas.

(5cii)
(i) Coal gas --> Importsmt gaseous fuel.
(ii) Coke --> Used as a reducing agent.

(5di)
(i) Calcium tetraoxosulphate(vi)
(ii) Calcium hydrogentrioxocarbonate(iv)

(5dii)
(i) By boiling.
(ii) Addition of caustic Soda.

(5diii)
(i) Hard water contains dissolved minerals and therefore tastes better.
(ii) The calcium salts on hard water help animals to build strong teeth and bones.


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ANSWER LOADING......
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