JAMB Chemistry 2005
Questions & Answers

The atomic number of an element is equal to the number of protons in its nucleus. Since the element has 17 protons, its atomic number is 17. The number of neutrons (18) contributes to the mass number but not the atomic number.

At STP, 1 mole of any ideal gas occupies 22.4 dm³. For 0.5 moles: Volume = 0.5 × 22.4 = 11.2 dm³. This applies to any gas, as the molar volume is independent of the gas’s identity.

In H₂SO₄, H has +1, O has -2. Let S be x. Equation: 2(+1) + x + 4(-2) = 0 → 2 + x - 8 = 0 → x = +6. Thus, sulfur’s oxidation number is +6.

Ethyne (C₂H₂) has a carbon-carbon triple bond, as each carbon shares three pairs of electrons to achieve a stable octet, characteristic of alkynes.

A negative enthalpy change (ΔH < 0) indicates that the reaction releases heat to the surroundings, making it exothermic.

Assume 100 g of compound. Moles: C = 40/12 = 3.33, H = 6.67/1 = 6.67, O = 53.33/16 = 3.33. Ratio: 3.33:6.67:3.33 = 1:2:1. Empirical formula = CH₂O.

Aluminium (Group 13) has three valence electrons, forming AlCl₃ by sharing or transferring electrons with three chlorine atoms. Other options form NaCl, MgCl₂, or SiCl₄.

NaOH is a strong base, fully dissociating to give [OH⁻] = 0.01 M. pOH = -log(0.01) = 2. pH = 14 - pOH = 14 - 2 = 12.

In dilute NaCl solution, water is preferentially oxidized at the anode, producing oxygen gas (2H₂O → O₂ + 4H⁺ + 4e⁻). Chlorine is produced in concentrated solutions.

H% = 100 - 85.7 = 14.3%. Moles in 100 g: C = 85.7/12 ≈ 7.14, H = 14.3/1 = 14.3. Ratio: 7.14:14.3 = 1:2 → CH₂. Molar mass of (CH₂)ₙ = 42 → 14n = 42 → n = 3. Molecular formula = C₃H₆.

For a first-order reaction, the half-life (t₁/₂ = ln(2)/k) is constant and independent of the initial concentration, a defining feature.

CH₃CH₂OH has two carbon atoms and an -OH group, making it an alcohol named ethanol (ethane + -ol) per IUPAC nomenclature.

Bauxite (Al₂O₃·2H₂O) is the primary ore of aluminium, extracted via the Bayer process and electrolysis in the Hall-Héroult process.

H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O. Moles of KOH = 0.1 × 0.025 = 0.0025. Moles of H₂SO₄ = 0.0025/2 = 0.00125. Volume = 0.00125/0.2 = 0.00625 dm enregistrer³ = 6.25 cm³.

Carbon dioxide (CO₂) traps infrared radiation, contributing to the greenhouse effect and global warming, unlike N₂, O₂, or Ar.

A molecule with four bonding pairs and no lone pairs (e.g., CH₄) adopts a tetrahedral shape, with bond angles of 109.5°, per VSEPR theory.

The Haber process combines nitrogen (N₂) and hydrogen (H₂) to produce ammonia (NH₃) under high pressure and temperature with a catalyst.

Molar mass of NaCl = 23 + 35.5 = 58.5 g/mol. Mass of 2 moles = 2 × 58.5 = 117 g.

Complete combustion of propane (C₃H₈ + 5O₂ → 3CO₂ + 4H₂O) produces carbon dioxide and water. Incomplete combustion may yield CO or soot.

E°cell = E°cathode - E°anode. Cu²⁺/Cu (cathode, +0.34 V), Zn²⁺/Zn (anode, -0.76 V). E°cell = 0.34 - (-0.76) = 1.10 V.

Isomers have the same molecular formula. 2-Methylbutane (C₅H₁₂) is a branched isomer of pentane, unlike butane (C₄H₁₀), hexane (C₆H₁₄), or propane (C₃H₈).

Burning fossil fuels in vehicles and power plants releases nitrogen oxides (NOₓ), contributing to air pollution and acid rain.

Calcium carbonate (CaCO₃) neutralizes excess stomach acid (HCl) by forming CaCl₂, H₂O, and CO₂, relieving indigestion.

In CO₂ (O=C=O), the carbon forms two double bonds, requiring two hybrid orbitals. This corresponds to sp hybridization, with a linear geometry.

Ionic compounds conduct electricity when molten or dissolved due to mobile ions, unlike in the solid state. They typically have high melting points and varying solubility.

Ethene (C₂H₄) undergoes electrophilic addition with Br₂ in water, forming 1,2-dibromoethane (C₂H₄Br₂), decolorizing the bromine solution.

Phosphorus is essential for plant growth, used in fertilizers (e.g., phosphates) to promote root development and energy transfer.

For 2A + B ⇌ C + D, Kc = [C][D]/[A]²[B], where products are over reactants, with coefficients as exponents.

Iron is a transition metal (d-block element) with partially filled d-orbitals, exhibiting variable oxidation states and catalytic properties.

Percentage yield = (actual yield/theoretical yield) × 100 = (4.0/5.0) × 100 = 80%.

Fractional distillation separates crude oil into fractions (e.g., petrol, kerosene) based on differences in boiling points.

Increasing pressure favors the side with fewer moles of gas. Here, 4 moles (N₂ + 3H₂) form 2 moles (2NH₃), so the equilibrium shifts to the right.

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. Carbon dioxide is released, identifiable by effervescence and limewater test.

CH₃COOH (ethanoic acid) contains the carboxyl group (-COOH), characteristic of carboxylic acids.

Biogas, produced from organic waste, is renewable, unlike fossil fuels (coal, natural gas, petroleum), which are finite.

Molar mass of NaOH = 23 + 16 + 1 = 40 g/mol. Moles = 4/40 = 0.1. Volume = 250 cm³ = 0.25 dm³. Molarity = 0.1/0.25 = 0.4 M.

Chlorine, being a halogen, has a high electronegativity (3.0 on the Pauling scale) due to its strong attraction for electrons, unlike Na, Mg, or Al.

A catalyst lowers the activation energy by providing an alternative reaction pathway, increasing the rate without being consumed.

Anhydrous calcium chloride (CaCl₂) is commonly used as a drying agent due to its ability to absorb moisture effectively.

Chlorofluorocarbons (CFCs) release chlorine atoms in the stratosphere, which catalyze the breakdown of ozone (O₃), depleting the ozone layer.