neco 2019 chemistry | Practical

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Question 1 View Details
A student performed a flame test on three unknown solid samples labelled X, Y and Z. The samples were placed separately in a clean platinum wire loop and introduced into a non‑luminous Bunsen‑burner flame. The following observations were recorded: - Sample X produced a bright orange‑red flame. - Sample Y produced a lilac (pale violet) flame. - Sample Z produced a green flame. The student used the standard flame‑colour chart for metal ions.
Question Parts
(a)
State the flame colour observed for each of the three samples.
(b)
Identify the metal ion most likely responsible for the flame colour of each sample, using the periodic table.
(c)
Suggest one possible source of error that could affect the observed flame colour and state how it can be minimised.
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Question 2 View Details
A student carried out a gravimetric determination of the atomic mass of magnesium by converting a weighed piece of magnesium ribbon into magnesium oxide. Data recorded: - Mass of clean crucible with lid (empty) = 45.000 g - Mass of crucible with lid and magnesium ribbon before heating = 45.1500 g - After heating the ribbon in the crucible until a constant mass was obtained, the mass of crucible with lid and product = 45.2100 g The reaction is: Mg (s) + ½ O₂ (g) → MgO (s) Assume the molar mass of oxygen = 16.00 g·mol⁻¹. Using the data above, answer the following:
Question Parts
(a)
Calculate the mass of magnesium oxide (MgO) produced.
(b)
Determine the mass of oxygen that combined with magnesium.
(c)
Calculate the number of moles of magnesium present in the original sample.
(d)
Using the experimental data, calculate the atomic mass of magnesium. Compare your result with the accepted value (24.31 g·mol⁻¹) and comment on the accuracy of the experiment.
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Question 3 View Details
A white crystalline solid sample was examined in the chemistry laboratory. The following observations were recorded: - The solid is soluble in distilled water, producing a clear solution. - The solution conducts electricity with a measured conductivity of 1.2 × 10⁻² S·cm⁻¹. - The solid has a melting point of 801 °C. - When a few drops of the solution are treated with silver nitrate solution, a white precipitate forms immediately. Based on the above experimental data, answer the questions that follow.
Question Parts
(a)
Identify the type of chemical bonding present in the solid sample.
(b)
Provide two independent pieces of experimental evidence from the observations that support your identification in part (a).
(c)
Suggest one further practical test that could be performed to confirm the ionic nature of the compound and briefly describe the expected observation.
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Question 4 View Details
A student carried out a titration to determine the molar mass of an unknown monoprotic acid (solid). The procedure and data are as follows: 1. The NaOH solution used in the titration was standardised using 0.2000 g of potassium hydrogen phthalate (KHP, molar mass = 204.22 g mol⁻¹). The volume of NaOH required to reach the phenolphthalein endpoint was 21.80 mL. 2. A sample of the unknown acid weighing 0.542 g was dissolved in distilled water and titrated with the same NaOH solution. The volume of NaOH used to reach the endpoint was 23.45 mL. 3. The theoretical molar mass of the pure acid is known to be 120.1 g mol⁻¹. Using the data above, answer the following questions:
Question Parts
(a)
Calculate the actual concentration (M) of the NaOH solution after standardisation with KHP.
(b)
Using the concentration obtained in part (a), determine the molar mass of the unknown acid.
(c)
Calculate the percent purity of the sample, assuming the impurity is inert and the theoretical molar mass of the pure acid is 120.1 g mol⁻¹.
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