Chemistry 2018 Jamb Examination
Chemistry 2018 Jamb Examination
Question 71
Calculate the percentage composition of oxygen in calcium trioxocarbonate(IV) [Ca=40, C=12, O=16]
The molar mass of calcium trioxocarbonate(IV), CaCO₃, is calculated as:\[M_r = 40 + 12 + (16 \times 3) = 100\]Percentage composition of oxygen:\[\dfrac{48}{100} \times 100 = 48\%\]Therefore, oxygen constitutes 48% of calcium trioxocarbonate(IV).
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Question 72
Which of these alloys contains copper?
Bronze is an alloy made primarily of copper and tin. It possesses good strength and corrosion resistance, making it useful in statues, coins, and mechanical parts. Solder is made of tin and lead, steel contains iron and carbon, and permalloy consists mainly of nickel and iron. Therefore, bronze is the alloy containing copper.
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Question 73
If the molecular mass of tetraoxosulphate(VI) acid is 98, calculate its vapour density.
Vapour density (V.D.) is given by:\[V.D. = \dfrac{\text{Molecular mass}}{2}\]For Hâ‚‚SOâ‚„:\[V.D. = \dfrac{98}{2} = 49\]Therefore, the vapour density of tetraoxosulphate(VI) acid is 49.
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Question 74
Sieving is a technique used to separate mixtures containing solid particles of
Sieving separates mixtures of solid particles based on size differences. The sieve allows smaller particles to pass through while retaining larger ones. This method is commonly used in separating flour from husks or sand from gravel. Hence, sieving is used for mixtures with particles of different sizes.
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Question 75
3H₂(g) + N₂(g) ⇔ 2NH₃(g); ΔH = -ve
In the reaction above, lowering of temperature will
In the reaction above, lowering of temperature will
The synthesis of ammonia is an exothermic reaction (ΔH negative). According to Le Chatelier’s principle, decreasing temperature favours the exothermic direction to oppose the change. Therefore, lowering the temperature shifts the equilibrium towards ammonia formation, favouring the forward reaction.
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Question 76
The situation obtained when a perfect gas expands into a vacuum is
When a perfect gas expands into a vacuum, no external work is done (since pressure is zero), and no heat is exchanged (adiabatic and free expansion). Therefore, ΔH = 0. However, the disorder of the system increases, leading to an increase in entropy (ΔS positive). Thus, TΔS is positive while ΔH is zero.
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Question 77
The general formula of alkanones is
Alkanones are ketones, and their general formula is R₂CO, where two alkyl groups are attached to a carbonyl group (C=O). For example, propanone (CH₃COCH₃) is an alkanone. RCHO represents aldehydes, RCOOH represents carboxylic acids, and RCOOR represents esters.
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Question 78
The alkanoic acid found in human sweat is
Human sweat contains propanoic acid (CH₃CH₂COOH), which contributes to its characteristic odour. It is a three-carbon carboxylic acid belonging to the alkanoic acid family. Ethanoic acid (CH₃COOH) is vinegar acid, and methanoic acid (HCOOH) occurs in ant venom. Thus, the alkanoic acid in sweat is propanoic acid.
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Question 79
The elements in the periodic table are listed in order of increasing
The modern periodic table is arranged in order of increasing atomic number, which represents the number of protons in an atom. This arrangement reflects the periodic recurrence of chemical properties. Earlier versions were based on atomic mass, but anomalies led to the adoption of atomic number as the defining property.
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Question 80
H₂S(g) + Cl₂(g) → 2HCl(g) + S(g)
In the reaction above, the substance that is reduced is
In the reaction above, the substance that is reduced is
Reduction involves the gain of electrons or decrease in oxidation number. In this reaction, chlorine gas (Clâ‚‚) is reduced to hydrogen chloride (HCl) as it changes from oxidation state 0 in Clâ‚‚ to -1 in HCl.
Meanwhile, hydrogen sulphide (Hâ‚‚S) is oxidised to sulphur. Therefore, the substance that is reduced is chlorine (Clâ‚‚).
Meanwhile, hydrogen sulphide (Hâ‚‚S) is oxidised to sulphur. Therefore, the substance that is reduced is chlorine (Clâ‚‚).
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