ijmb model questions vol1 2025 chemistry | Practical

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Question 1 View Details
A mass‑spectrometer was used to analyse a sample of chlorine gas. The instrument gave two peaks at mass‑to‑charge ratios (m/z) of 35 and 37 with relative intensities of 75 and 25 respectively. Using this data, answer the following questions.
Question Parts
(a)
Calculate the average atomic mass of chlorine from the experimental data.
(b)
State the percentage natural abundance of each isotope.
(c)
The accepted standard atomic mass of chlorine is 35.45 amu. Discuss why the experimental value (from part a) differs from the standard value.
(d)
Suggest two practical precautions that could improve the accuracy of the atomic‑mass determination.
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Question 2 View Details
A flame‑photometer was used to analyse an unknown aqueous solution for the presence of alkali metal ions. The instrument was first calibrated using standard potassium chloride (KCl) solutions as follows: | Standard concentration (ppm K⁺) | Flame intensity (arbitrary units) | |----------------------------------|------------------------------------| | 0.5 | 15 | | 1.0 | 25 | | 1.5 | 35 | | 2.0 | 45 | The calibration data are linear and pass through the points given. The unknown solution gave a flame intensity of 45 units and produced a lilac flame colour. Answer the following questions.
Question Parts
(a)
Identify the metal ion most likely responsible for the observed lilac flame.
(b)
Determine the concentration of K⁺ in the unknown solution using the calibration data.
(c)
List two possible sources of interference that could affect the flame‑photometric determination of potassium.
(d)
Suggest two procedural improvements to minimise the interferences identified in part c.
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Question 3 View Details
A laboratory investigation was carried out to determine the type of chemical bonding present in three different substances. The following observations were recorded: 1. **Sample A** – a solid copper wire was connected to a digital multimeter. The resistance measured was 0.02 Ω. 2. **Sample B** – a dry crystalline solid of refined sucrose (table sugar) was placed between the same electrodes. No current was recorded (infinite resistance). 3. **Sample C** – 25 mL of an aqueous solution prepared by dissolving an unknown white solid in 100 mL of distilled water. The solution was clear and colourless. Conductivity of the solution was measured as 1.20 S m⁻¹. When a few drops of 0.1 M AgNO₃ solution were added, a white precipitate formed instantly. Using the data above, answer the questions that follow.
Question Parts
(a)
Identify the type of bonding present in each of the three samples (A, B and C).
(b)
Provide a brief justification for each identification, referring to the observations recorded.
(c)
State one possible source of error that could affect the identification of the bonding type for any of the samples.
(d)
Suggest one precaution that should be observed when performing the conductivity test.
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Question 4 View Details
A sample of impure calcium carbonate (CaCO₃) was analysed gravimetrically to determine its percent purity. Procedure (summarised): 1. A 1.250 g portion of the impure solid was weighed into a crucible. 2. The sample was treated with excess dilute H₂SO₄ to convert all CaCO₃ to calcium oxalate, which was then filtered, washed and dried. 3. The calcium oxalate precipitate was ignited in a muffle furnace at 900 °C until a constant mass was obtained. The product of ignition is calcium oxide (CaO). 4. After cooling in a desiccator, the mass of CaO was recorded as 0.560 g. Using the data above, answer the following questions.
Question Parts
(a)
Write the balanced chemical equation for the thermal decomposition of calcium carbonate to calcium oxide.
(b)
Calculate the number of moles of CaO obtained from the sample.
(c)
From the moles of CaO, determine the theoretical mass of CaCO₃ that would have been present in the original sample.
(d)
Calculate the percent purity of the impure CaCO₃ sample.
(e)
State two possible sources of error that could affect the accuracy of the percent purity obtained.
(f)
Suggest one improvement to the experimental procedure that would minimise the errors identified in part (e).
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