All 40 questions from the Joint Admissions and Matriculation Board (JAMB) Chemistry 2023 Objective paper, with the correct answer and a full explanation for each. Free, no signup needed.
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Given that the difference in electronegativity of elements X and Y is 2.2, the bond that will be formed between them is
Electronegativity difference >1.7 → ionic bond. 0.5–1.7 → polar covalent. <0.5 → non-polar covalent. A difference of 2.2 gives an ionic bond.
The radiation used in studying the arrangement of particles in giant organic molecules is
X-ray crystallography is the technique used to study atomic arrangements in giant organic molecules. Crystals are exposed to X-rays and the diffraction patterns are analysed.
i. CaCO₃ ii. Ca(HCO₃)₂ iii. CaSO₄ — Which of the above causes temporary water hardness if present in water?
Ca(HCO₃)₂ causes temporary hardness (removed by boiling). CaSO₄ causes permanent hardness. CaCO₃ is insoluble and does not cause hardness.
The dispersion of a liquid in a liquid medium will give
Emulsion: liquid dispersed in liquid. Sol: solid in liquid. Aerosol: liquid/solid in gas. Fog: liquid droplets in gas.
The respective basicities of HNO₃, HCl, CH₃COOH and H₂SO₄ are
Basicity = number of replaceable H⁺ ions per molecule. HNO₃ = 1, HCl = 1, CH₃COOH = 1 (only the –COOH proton), H₂SO₄ = 2.
C(s) + H₂O(g) → CO(g) + H₂(g) — The oxidising agent in the reaction above is
Carbon is oxidised (0 → +2 in CO), so it is the reducing agent. Hydrogen in H₂O is reduced (+1 → 0 in H₂), so H₂O is the oxidising agent.
How many coulombs of electricity will be needed to deposit 22.22 g of silver?
Ag⁺ + e⁻ → Ag. Q = (mass/M) × F × n = (22.22/108) × 96500 × 1 = 0.2057 × 96500 = 19853.98 ≈ 19854 C.
A compound that gives a brick-red colour to a non-luminous flame is likely to contain
Flame test: Ca²⁺ → brick-red; Na⁺ → golden yellow; K⁺ → lilac; Cu²⁺ → blue-green; Ba²⁺ → green; Li⁺ → crimson red.
The general formula of haloalkanes where X represents the halide is
Haloalkanes are derived from alkanes (CₙH₂ₙ₊₂) by replacing one H with halogen X. Formula = CₙH₂ₙ₊₁X.
Which of the following is generated when ethyne is passed through a hot tube containing an organo-nickel catalyst?
3C₂H₂ →(organo-Ni, Δ)→ C₆H₆. Three molecules of ethyne trimerize to form benzene.
The petroleum fraction used in heating furnaces in industries is
Diesel oil has a high energy content and suitable combustion characteristics, making it suitable for industrial heating furnaces.
What is the molar mass of a gas if 2.0 g of the gas occupies a volume of 400 cm³ at STP?
Moles = 400/22400 = 0.01786 mol. Molar mass = mass/moles = 2.0/0.01786 = 112 g mol⁻¹.
The following factors have no effect on the value of the equilibrium constant EXCEPT
Only temperature changes the equilibrium constant (K). Pressure and concentration change the equilibrium position but not the value of K.
What volume of CO is produced by reacting excess carbon with 5 dm³ of oxygen?
2C + O₂ → 2CO. 1 mole O₂ produces 2 moles CO. By Gay-Lussac's law, 5 dm³ O₂ produces 10 dm³ CO.
The shape of the carbon(iv) oxide molecule is
CO₂ has two C=O double bonds with no lone pairs on carbon. VSEPR theory gives linear geometry (180°).
What type of bond can be formed between two elements with electronic structures 1s²2s²2p⁶ (oxygen group) and 1s²2s²2p⁴?
Both elements are non-metals (no electropositive element). Neither can lose electrons easily to form an ionic bond. They share electrons to form a covalent bond.
When water is added to anhydrous cobalt(ii) chloride (COCl₂), the colour changes from
Anhydrous CoCl₂ is blue. When hydrated (water added), it turns pink/red: CoCl₂ + 6H₂O → CoCl₂·6H₂O (pink). Anhydrous CuSO₄ changes white → blue on hydration.
The following are salts EXCEPT
Zn(NO₃)₂, Na₂CO₃ and MgCl₂ are salts. CaO is a base (metal oxide). Bases are not salts — they are oxides, hydroxides, or other compounds that react with acids.
In an electrochemical cell, polarisation is caused by
Hydrogen bubbles form on the anode surface during electrolysis, increasing cell resistance and reducing EMF. This is polarisation, prevented by depolarisers like MnO₂.
A metal that forms soluble trioxosulphate(iv) salt is
Generally, all sodium, potassium and ammonium salts are soluble in water. Potassium sulphite (K₂SO₃) is soluble.
2-methylpropan-2-ol is a/an
2-methylpropan-2-ol: the carbon bearing –OH is bonded to three other carbon atoms (three methyl groups). This is a tertiary alcohol.
The decarboxylation of ethanoic acid will produce
Decarboxylation of RCO₂H gives RH + CO₂. For ethanoic acid (CH₃COOH): CH₃COOH → CH₄ + CO₂. The alkane produced is methane.
Which of the following qualifies elements in the same period?
Elements in the same period have the same number of electron shells (energy levels). Elements in the same group have the same number of valence electrons.
A substance is said to be impure if
Pure substances have sharp, narrow melting point ranges. Impurities broaden and depress the melting point range. A wide melting range indicates impurity.
How many isomers does C₃H₆Cl₂ have?
C₃H₆Cl₂ has four structural isomers: 1,1-dichloropropane; 2,2-dichloropropane; 1,2-dichloropropane; and 1,3-dichloropropane.
Which of the following is non-biodegradable?
Plastics (like polyethene, PVC) cannot be broken down by micro-organisms and persist in the environment. Wood, paper and organic waste are biodegradable.
Which of the following is a fine chemical?
Fine chemicals are produced in small quantities to high purity (e.g. drugs, dyes, vitamins). Heavy chemicals like H₂SO₄, NaOH and NH₃ are produced in large quantities.
How many unpaired electrons are in the p-orbitals of an oxygen atom?
O (Z=8): 1s²2s²2p⁴. The 2p subshell has 4 electrons: 2px↑↓, 2py↑, 2pz↑. There are 2 unpaired electrons.
The radioactive emission with the least ionisation power is
Ionising power: α > β > γ. Alpha particles have the highest ionising power but lowest penetrating power. Gamma rays have the highest penetrating power but lowest ionising power.
How many atoms are present in 48 g of magnesium?
Moles of Mg = 48/24 = 2 mol. Atoms = 2 × 6.02 × 10²³ = 1.204 × 10²⁴.
50 cm³ of gas was collected over water at 10°C and 765 mmHg. Calculate the volume at STP if the SVP of water at 10°C is 5 mmHg.
Actual pressure of gas = 765 − 5 = 760 mmHg. At STP: T₁=283 K, V₁=50 cm³, P₁=760 mmHg; T₂=273 K, P₂=760 mmHg. V₂ = (50 × 273)/283 = 48.23 cm³.
Group VII elements are
Group VII (halogens) are diatomic, highly electronegative, good oxidising agents. They are NOT electron donors or electropositive. They accept electrons (reduction).
A substance that absorbs water when exposed to the atmosphere to become ultimately wet or sticky is
Deliquescent substances absorb water from the air until they dissolve (e.g. NaOH, CaCl₂). Hygroscopic substances absorb moisture but don't fully dissolve. Efflorescent substances lose water of crystallisation.
This illustrates the law of
The law of multiple proportion states that when two elements combine in more than one ratio, the different masses of one element that combine with a fixed mass of the other are in simple whole-number ratios.
NaOH + NH₄Cl → NaCl + H₂O + Y. Identity Y in the reaction above.
NaOH + NH₄Cl → NaCl + H₂O + NH₃. A base heated with an ammonium salt produces ammonia. Ammonia is produced in the laboratory when a base is heated with an ammonium salt.
Due to its high reactivity, sodium is usually stored in
Sodium is stored under paraffin oil (mineral oil) to prevent contact with air (O₂) and moisture, which would cause it to react violently.
The ability of an element to form long chains with its own atoms is referred to as
Catenation is the property of an atom forming long chains/rings by bonding with its own atoms. Carbon is the best example — it forms millions of organic compounds.
The miscibility of ethanol with water is due to the presence of
Ethanol (CH₃CH₂OH) is miscible with water because the –OH (hydroxyl) group forms hydrogen bonds with water molecules, enabling complete mixing.
Which of the following statements about thermoplastic material is correct?
Thermoplastics soften and melt on heating (allowing reshaping), then harden on cooling. This is reversible. Thermosets do not soften on heating — they char or decompose.
The radioactive emission with the least ionisation power is
Ionising power order: α > β > γ. Gamma rays have the least ionising power but greatest penetrating power. Alpha particles have the highest ionising power.
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