neco 2025 chemistry | Practical

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Question 1 View Details
A student determines the molar mass of magnesium by reacting weighed pieces of the metal with excess hydrochloric acid and collecting the hydrogen gas over water. The experiment is carried out at 25 °C and the atmospheric pressure is 760 mmHg. The vapour pressure of water at this temperature is 23 mmHg. The data obtained are shown in the table below. | Trial | Mass of Mg (g) | Volume of H₂ collected (L) | |-------|----------------|----------------------------| | 1 | 0.500 | 0.550 | | 2 | 0.750 | 0.825 | | 3 | 1.000 | 1.100 | Using the data, answer the following:
Question Parts
(a)
Calculate the number of moles of H₂ collected in each trial.
(b)
Determine the experimental molar mass of magnesium for each trial.
(c)
Calculate the average experimental molar mass of magnesium from the three trials.
(d)
State two possible sources of error that could affect the result obtained.
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Question 2 View Details
A student carries out a systematic qualitative analysis of an unknown aqueous metal salt solution. The following observations are recorded during the test sequence: 1. Addition of dilute HCl produces no visible change. 2. Addition of NaOH results in a blue precipitate which dissolves on adding excess NaOH. 3. Addition of a mixture of NH₄Cl and NH₄OH does not cause the precipitate to re‑appear. 4. Addition of potassium ferrocyanide solution gives a reddish‑brown precipitate. Based on the observations, answer the questions below:
Question Parts
(a)
Identify the metal cation present in the unknown solution and state its group (period and family) in the periodic table.
(b)
Write the balanced chemical equation for the reaction of the identified cation with NaOH that forms the blue precipitate.
(c)
Write the balanced chemical equation for the reaction of the identified cation with potassium ferrocyanide that yields the reddish‑brown precipitate.
(d)
Suggest one source of error that could lead to an incorrect identification of the cation.
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Question 3 View Details
A student is to investigate the nature of bonding in three solid compounds: (i) sodium chloride (NaCl), (ii) copper(II) sulfate pentahydrate (CuSO4·5H2O), and (iii) iodine (I2). Using the laboratory facilities available, the student will carry out an electrical conductivity test of each compound in distilled water and record observations.
Question Parts
(a)
Describe the complete procedure for testing the electrical conductivity of each compound when dissolved in distilled water. Include the apparatus required and the steps to be followed.
(b)
State the type of bonding (ionic, covalent or metallic) you would expect for each compound based on the conductivity observations.
(c)
Explain why the observed conductivities differ among the three compounds, relating your answer to the nature of bonding and the particles formed in solution.
(d)
Identify one practical limitation of using the conductivity test alone to determine the type of bonding and suggest an additional test that could complement it.
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Question 4 View Details
A 5.00 g sample of a solid suspected to be calcium carbonate (CaCO₃) is reacted with excess hydrochloric acid. The carbon dioxide gas produced is collected over water at 25 °C. The volume of gas collected is 0.845 L. Atmospheric pressure is 760 mmHg and the vapour pressure of water at 25 °C is 23.8 mmHg. Using this data, determine the amount of CaCO₃ present in the sample and evaluate its percent purity.
Question Parts
(a)
Write the balanced chemical equation for the reaction between calcium carbonate and hydrochloric acid.
(b)
Calculate the number of moles of dry CO₂ gas collected, correcting for the water vapour pressure. Use R = 0.0821 L·atm·mol⁻¹·K⁻¹.
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