WAEC 2021 - CHEMISTRY ANSWER


CHEMISTRY OBJ
1-10: CDBAAAACAD
11-20: CBBCDCCADB
21-30: CACCABACBC
31-40: DB-BDCBABD
41-50: CBBCCABDDA

(1a)
A molecular formula consists of the chemical symbols for the constituent followed by numeric subscripts showing the number of atoms of each element present while structural formula consists of symbols for the atoms connected by short lines that represent chemical bonds.

(1b)
(i) Screening effect
(ii) Size of atom
(iii) Nuclear change

(1c)
(i) The reaction must proceed both forward and backward i.e reversible
(ii) All the reactants and products are present at equilibrium

(1d)
(i) B
(ii) It has the least ionization energy among the three

(1e)
Graham's law of diffusion states that the rate of diffusion of a gas is inversely proportional to the square root of their densities provided other conditions remained constant

(1f)
(i) Na2SO4, Mg(NO3)2
(ii) CaCO3

(1g)
(i) Protein - condensational polymer
(ii) Perspex - additional polymer
(iii) Nylon - Condensation polymer

(1h)
Atomic radius can be defined as the half distance between the nucleus of two covalently bonded atoms.

(1i)
Ethanol has a higher boiling point than propane because it has stronger force of attraction than propane which has a weak van der waal's force.

(1j)
(i) It aids the digestion of food
(ii) It aids the action of enzymes
(iii) It controls the availablity of nutrients

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(2ai)
(i) They have the same method of preparation
(ii) They have the same chemical properties

(2aii)
(i) Ethane has a single bond while Ethene has a double bond
(ii) They belong to different homogeneous series
(iii) They have different molecular formula and structural formula

(2bi)
HCl

(2bii)
-It is colourless
-It has an irritating odour

(2biii)
2NaCl(aq) + H2SO4(aq) ---> Na2SO4(aq) + 2HCl(g)

(2ci)
Hydrocarbons are compounds having hydrogen and carbon atoms only. Examples are alkanes, alkenes, alkynes etc

(2cii)
(i) Coal
(ii) Crude oil

(2ciii)
C H
83% 100.83
83/12 17/1
6.92/6.92 17/6.92
1 2

Empirical formula = CH2

(2d)



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(3ai)
H2SO4 (aq) + 2KOH(aq) ---> K2SO4(aq) + 2H2O(l)

(3aii)
Given, Va = 25.0cm³, Vb = 24.0cm³, Cb = 0.15mol/dm³, Ca =?, Na = 1 , Nb = 2

Using:
CaVa/CbVb = Na/Nb
Ca = CbVbNa/VaNb
Ca = 0.15 × 24.0 × 1/25.0×2
Ca = 3.6/50 = 0.072mol/dm³

(3bi)
(i) 2Mg(s) + CO2(g) ---> 2MgO(aq) + C(g)
(ii) This is because magnesium can reduce carbon (iv) oxide to black carbon and producing magnesium oxide

(3bii)
Mg(NO3)2
M of Mg(NO3)2 = 24+(2×14) + (6×16)
= 24 + 28 + 96
= 148g/mol
% of N = 28/148 × 100/1
N = 18.9%

(3ci)
(i) Reaction with KMnO4
(ii) Reaction with bromine water

(3cii)
It turns them colourless

(3d)
Boil the vegetable oil in sodium hydroxide solution, it breaks down releasing the organic acid and the alkanol. The process is known as saponification. The organic ester is immediately neutralized by the sodium hydroxide solution to form the sodium salt of the organic acid which is soap.

(3e)
Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. The higher the value of the electronegativity, the more strongly that element attracts the shared electrons.


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(5)