ijmb model questions vol1 2025 chemistry | Essay

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Question 1 View Details
The diagram shows two naturally occurring isotopes of a neutral element X. The nucleus of each isotope is illustrated with the number of protons (Z) and neutrons (N). The electron shells are drawn showing the distribution of electrons. The isotopic data are: - Isotope‑1: mass number 35, relative abundance 75 % - Isotope‑2: mass number 37, relative abundance 25 % The element has atomic number 17 (i.e., 17 protons).
Question Parts
(a)
State the atomic number and the symbol of the element.
(b)
Calculate the number of neutrons present in each isotope.
(c)
Write the ground‑state electron configuration of the element using the noble‑gas notation.
(d)
Determine the average (relative) atomic mass of the element from the isotopic data given.
(e)
Explain why the calculated average atomic mass is not a whole number, relating your answer to the concept of isotopic abundance.
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Question 2 View Details
The diagram shows a Bohr model of an unknown element X. The nucleus is labelled X. Three concentric electron shells are drawn. The innermost shell contains 2 electrons, the second shell contains 8 electrons and the outermost shell contains 1 electron. The nucleus is annotated as containing 7 neutrons.
Question Parts
(a)
Determine the atomic number (Z) and the mass number (A) of element X.
(b)
Write the ground‑state electron configuration of X (i) using Bohr shell notation and (ii) using the modern orbital notation. State the chemical symbol of the element.
(c)
Sodium commonly loses its outermost electron to form an ion. (i) Write the ionic symbol of the ion formed and its charge. (ii) State the new electron configuration of the ion using modern notation.
(d)
Using periodic trends, compare the metallic character of element X with that of magnesium (Mg, Z = 12). State whether X is more metallic, less metallic, or about the same, and justify your answer with reference to their positions in the periodic table.
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Question 3 View Details
The diagram shows the Lewis structure of chlorine nitrite (ClNO₂). All atoms, bonds and lone pairs are clearly labelled.
Question Parts
(a)
Calculate the formal charge on each atom (Cl, N, and each O) in the molecule.
(b)
Using VSEPR theory, predict the molecular geometry around the nitrogen atom and give the approximate bond angle(s).
(c)
State whether the molecule is polar or non‑polar and justify your answer based on the geometry and the distribution of formal charges.
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Question 4 View Details
The diagram shows the structural representation of copper(II) sulfate pentahydrate, CuSO4·5H2O. The copper ion (Cu²⁺) is surrounded by four water molecules in an octahedral coordination sphere and is ionically bonded to a sulfate ion (SO4²⁻). Five additional water molecules are present as water of crystallisation attached to the lattice. Using the information from the diagram, answer the following questions:
Question Parts
(a)
Calculate the number of moles present in a 2.00 g sample of CuSO4·5H2O.
(b)
From the sample in part (a), determine the number of moles of water of crystallisation that would be released on heating the hydrate to obtain anhydrous CuSO4.
(c)
Calculate the mass of anhydrous CuSO4 that remains after complete removal of the water of crystallisation from the sample in part (a).
(d)
Determine the mass percent of copper (Cu) in the original CuSO4·5H2O sample.
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Question 5 View Details
A mixture containing 12.0 g of calcium carbonate (CaCO₃) and 10.0 g of magnesium oxide (MgO) is heated strongly in a crucible. Calcium carbonate decomposes according to CaCO₃ → CaO + CO₂, while magnesium oxide remains unchanged. The calcium oxide formed reacts with excess aqueous hydrochloric acid according to CaO + 2 HCl → CaCl₂ + H₂O. After filtration, the calcium chloride solution is treated with excess sodium carbonate solution, precipitating calcium carbonate: CaCl₂ + Na₂CO₃ → CaCO₃↓ + 2 NaCl. The precipitate is dried and weighed, giving 5.00 g of CaCO₃.
Question Parts
(a)
Identify the limiting reagent in the decomposition of calcium carbonate and calculate the theoretical mass of CaO that could be produced from the mixture.
(b)
Using the 5.00 g of CaCO₃ recovered, calculate the overall percent yield for the sequence CaCO₃ → CaO → CaCl₂ → CaCO₃.
(c)
Determine the mass of magnesium oxide that remains unchanged after the heating step.
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Question 6 View Details
A 2.00 mol sample of nitrogen gas (N₂) at 25 °C and 1.00 atm is placed in a sealed rigid container of volume 5.00 L. The gas is then compressed isothermally to a final pressure of 5.00 atm. Afterwards, the temperature is raised at constant volume until the pressure reaches 10.0 atm.
Question Parts
(a)
Calculate the work done on the gas during the isothermal compression.
(b)
Determine the final temperature of the gas after the constant‑volume heating step.
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Question 7 View Details
A buffer solution is prepared by dissolving 0.250 mol of acetic acid (CH₃COOH, Ka = 1.8 × 10⁻⁵) and 0.150 mol of sodium acetate (CH₃COONa) in enough water to make 1.00 L of solution.
Question Parts
(a)
Calculate the pH of the buffer solution.
(b)
0.050 mol of 0.10 M HCl is added to the buffer (assume the volume change is negligible). Determine the new pH and explain the direction of the pH change.
(c)
Recommend a suitable indicator for titrating this buffer with NaOH and justify your choice based on the expected pH range of the titration.
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Question 8 View Details
The solubility of silver chloride (AgCl) in water is 1.33 × 10⁻⁵ mol L⁻¹ at 25 °C and 1.90 × 10⁻⁵ mol L⁻¹ at 60 °C.
Question Parts
(a)
Calculate the solubility product constant (Ksp) of AgCl at 25 °C and at 60 °C.
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Question 9 View Details
Potassium permanganate is used to oxidise oxalic acid in an acidic medium. The reaction is carried out in a solution containing sulphuric acid as the source of acidity. The balanced molecular equation, the masses of products formed and the standard cell potential are required.
Question Parts
(a)
Write the oxidation half‑reaction for oxalic acid (C₂O₄²⁻ → CO₂) and the reduction half‑reaction for permanganate ion (MnO₄⁻) in acidic solution, showing the electrons transferred.
(b)
Combine the half‑reactions to obtain the overall balanced molecular equation for the reaction between potassium permanganate and oxalic acid in acidic solution, including the spectator ions (K⁺, H⁺, SO₄²⁻).
(c)
If 0.125 mol of KMnO₄ reacts completely, calculate the mass of carbon dioxide produced. (Molar mass of CO₂ = 44.01 g mol⁻¹)
(d)
Given the standard reduction potentials: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O E° = +1.51 V and C₂O₄²⁻ → 2CO₂ + 2e⁻ E° = +0.49 V, calculate the standard cell potential (E°cell) for the overall reaction written in part (b). State which half‑reaction occurs at the cathode and which at the anode.
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Question 10 View Details
A galvanic cell is set up with a copper electrode in CuSO₄ solution and a zinc electrode in ZnSO₄ solution. The cell notation is: Cu(s) | Cu²⁺ (0.010 M) || Zn²⁺ (0.10 M) | Zn(s). Standard reduction potentials are: Cu²⁺ + 2e⁻ → Cu E° = +0.34 V; Zn²⁺ + 2e⁻ → Zn E° = –0.76 V.
Question Parts
(a)
Write the overall cell reaction and identify which electrode functions as the anode and which as the cathode under standard conditions.
(b)
Using the concentrations given, calculate the cell potential at 25 °C with the Nernst equation.
(c)
If a current of 0.250 A is passed through the cell for 2.00 h, determine the mass of zinc that would be deposited on the zinc electrode (assume the cell operates as an electrolytic cell for zinc plating). Use F = 96 485 C mol⁻¹ and the molar mass of Zn = 65.38 g mol⁻¹.
(d)
Calculate the electrical energy (in kJ) consumed during the electrolysis in part (c), taking the cell potential obtained in part (b) as the operating voltage (assume it remains constant).
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