Chemistry Past Questions | West African Examinations Council (WAEC)

Question 41

The compound formed between ${}_{14}$

X and ${}_{16}$

Y is _______?

Options

A) XY

B) XY${}_2$

C) X${}_2$

Y

D) X${}_4$

Y${}_6$

The correct answer is B.

Question 42

The shape of a molecule of water is _______?

Options

A) Non-linear

B) Octahedral

C) Pyramidal

D) Tetrahedral

The correct answer is A.

Question 43

A metallic ion Z²⁺ with an inert gas structure contains 18 electrons. How many protons are contained in this ion?

Options

A) 20

B) 18

C) 16

D) 2

The correct answer is A.

Question 44

Which of the following pairs of elements forms amphoteric oxides?

Options

A) Be and Mg

B) Na and K

C) B and Al

D) Si and Pb

The correct answer is D.

Question 45

Which of the following pairs of compounds belongs to the same homologous series?

Options

A) C₃H₈ and C₃H₆

B) C₄H₁₀ and C₅H₁₀

C) C₂H₄ and C₄H₁₀

D) C₂H₆ and C₄H₁₀

The correct answer is D.

Explanation:

In organic chemistry, a homologous series is a series of compounds with the same functional group and similar chemical properties in which each successive member differs by CH₂.

Question 46

The following transition metal ions would be coloured in aqueous solution except __________.

Options

A) Cr³⁺

B) Fe³⁺

C) Mn³⁺

D) Sc³⁺

The correct answer is D.

Question 47

The gas given off when ethanol reacts with sodium is

Options

A) Carbon(IV)oxide

B) Hydrogen

C) Methane

D) Oxygen

The correct answer is B.

Explanation:

If a small piece of sodium is dropped into some ethanol, it reacts steadily to give off bubbles of hydrogen gas and leaves a colourless solution of sodium ethoxide, CH${}_3$

CH${}_2$

ONa. Sodium ethoxide is known as an alkoxide.

If the solution is evaporated carefully to dryness, the sodium ethoxide is left as a white solid.

2CH${}_3$

CH${}_2$OH + 2Na $\to$ 2CH${}_3$CH${}_2$ONa + H${}_2$

Question 48

Which of the following halogens is solid at room temperature?

Options

A) Bromine

B) Chlorine

C) Flourine

D) Iodine

The correct answer is D.

Question 49

The alkaline earth metals have similar chemical properties because __________.

Options

A) They are in the same period

B) Their salts are colourless

C) They have the same number of valence electrons

D) They are very reactive

The correct answer is C.

Explanation:

Element in the same group have the same number of valence electrons. Alkaline earth metals are the group 2 elements.
Teslim Memunat

Question 50

The number of unpaired electrons in an atom of an element ₈Q is __________.

Options

A) 2

B) 4

C) 6

D) 8

The correct answer is A.

Explanation:

In chemistry, an unpaired electron is an electron that occupies an orbital of an atom singly, rather than as part of an electron pair. Each atomic orbital of an atom (specified by the three quantum numbers n, l and m) has a capacity to contain a maximum of two electrons with opposite spins, an electron pair.

Electrons in an atom can be identified in the shells. The maximum number of electrons in a shell depends on the formula 2n${}^2$

, where n is the number of the shell. Each shell has orbitals, known as s, p, d and f. The first shell has one orbital, s, while the second shell has two orbitals, s and p. The number of orbitals increases with the number of shells.

To get the number of unpaired electrons, we must see the electron configuration. Oxygen's atomic number is 8, so it has eight protons and electrons. These eight electrons are distributed in the shells. The electron configuration is 1s${}^2$

2s${}^2$2p${}^4$. The maximum number of electrons in the second shell is 2n${}^2$ = 2 $\times$ 2${}^2$ = 8. In this case, total electrons in the second shell = 2 (in s orbital) + 4 (in p orbital) = 6. So, the number of unpaired electrons = 8 - 6 = 2.

Question 51

The bond formed when ammonia reacts with hydrogen ion to form ammonium ion is

Options

A) Covalent

B) Dative

C) Hydrogen bond

D) Ionic

The correct answer is B.

Question 52

To which group and period respectively does an element with 15 electrons belong?

Options

A) 3 and 3

B) 3 and 5

C) 5 and 3

D) 5 and 5

The correct answer is C.

Explanation:

(P) Arsenic is the element with 15 electrons which is found in Group 5 and Period 3.

Question 53

The shape of a graphite crystal is

Options

A) Tetrahedral

B) Pyramidal

C) Octahedral

D) Hexagonal

The correct answer is D.

Question 54

Which of the following oxides is ionic?

Options

A) P₄O₁₀

B) MgO

C) Al₂O₃

D) SO₂

The correct answer is B.

Question 55

Which of the following substances when boiled with aqueous solution of sodium hydroxide would be hydrolyzed?

I. Protein

Ii. Fat

Iii. Polythene

Options

A) i

B) ii

C) i and ii

D) ii and iii

The correct answer is C.

Question 56

Which of the following trioxonitrate (V) salts would decompose on heating to form a metal?

Options

A) Cu(NO₃)₂

B) AgNO₃

C) Pb(NO₃)₂

D) KNO₃

The correct answer is B.

Question 57

Consider the following reaction equation:
C₂H_4(g) + 3O_2(g) → 2CO_2(g) + 2H₂O_(g)

The volume of CO_2(g) produced at s.t.p when 0.05 moles of C₂H_4(g) was burnt in O_2(g) is __________. [Molar Volume of gas = 22.4dm³]

Options

A) 1.12dm³

B) 2.24dm³

C) 3.72dm³

D) 4.48dm³

The correct answer is A.

Explanation:

$\frac{volume}{molarvolume}$

= 0.05
= $\frac{volume}{22.4}$

= 0.05 × 22.4 = 1.12dm³

Question 58

The number of oxygen molecules present in 16.0g of the gas is __________.
[Avogadro's number = 6.02 × 10²³]

Options

A) 6.02 × 10²²

B) 6.02 × 10²³

C) 3.01 × 10²³

D) 1.51 × 10²³

The correct answer is C.

Explanation:

1 mole = Molar mass of gas = Avogadros number
32g of O₂ = 6.02 × 10²³
16g of O₂ = x
x = $\frac{16\times 6.02\times {10}^{2}3}{32}$

= 3.01 × 10²³

Question 59

Consider the following reaction equation:
SO₄^2-_(aq) + 2H⁺_(aq) + ye^- → SO₄^2-_(aq) + H₂O_(l).

The value of y in the equation is

Options

A) 2

B) 3

C) 4

D) 6

The correct answer is A.

Explanation:

From the reaction, the total charge on the left side is equal to the total charge on the right side.
-2 + 2 + y = -2
y = -2
The value of y is 2.

Question 60

The general gas equation was derived from _________.

Options

A) Boyle's and Gay Lussac's laws

B) Boyle's and Graham's laws

C) Boyle's and Charles' laws

D) Dalton's atomic theory

The correct answer is C.

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