neco 2023 chemistry | Practical

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Question 1 View Details
A sample of an unknown metal X is converted to its oxide by heating in air. The mass of the metal before heating is 1.250 g and the mass of the oxide obtained after heating is 1.950 g. It is known that the oxide formed has the empirical formula X₂O₃.
Question Parts
(a)
Write the balanced chemical equation for the oxidation of X.
(b)
Calculate the atomic mass of X (in g mol⁻¹). Give your answer to one decimal place.
(c)
State one source of experimental error that could affect the calculated atomic mass.
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Question 2 View Details
A student is investigating the concentration of magnesium ions in an unknown solution using a UV‑visible spectrophotometer at 285 nm. The Beer‑Lambert law applies and the absorbance is directly proportional to concentration, passing through the origin. The data obtained are shown below. | Standard solution (C, mol L⁻¹) | Absorbance (A) | |-------------------------------|----------------| | 0.20 | 0.120 | | 0.40 | 0.245 | | 0.60 | 0.368 | | 0.80 | 0.492 | The unknown solution gives an absorbance of 0.310.
Question Parts
(a)
Calculate the average slope (ΔA/ΔC) of the calibration curve using the four standard solutions.
(b)
Determine the concentration of Mg²⁺ in the unknown solution.
(c)
Suggest two possible sources of systematic error that could cause the calculated concentration to be higher than the true value.
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Question 3 View Details
A student is given a white solid sample suspected to be a transition‑metal chloride. The following procedures are carried out: (i) 0.50 g of the solid is dissolved in 25.0 cm³ of distilled water and the conductivity of the resulting solution is measured. (ii) A small amount of the solid is placed in a test tube, a few drops of concentrated HCl are added and the colour change is observed. (iii) A small quantity of the solid is subjected to a magnetic test using a suspended bar magnet. The conductivity reading obtained in step (i) is 120 µS·cm⁻¹, the addition of HCl produces a deep‑green solution and the solid is attracted to the magnet. Using this information, answer the questions below.
Question Parts
(a)
Calculate the molar concentration of the solution prepared in step (i). Assume the molar mass of the compound is 200 g·mol⁻¹.
(b)
Using the conductivity reading of 120 µS·cm⁻¹, state whether the compound behaves as an ionic or a covalent substance in aqueous solution. Justify your answer.
(c)
The addition of concentrated HCl produced a deep‑green solution and the solid was attracted to the magnet. Deduce: (i) the oxidation state of the metal ion, (ii) the number of unpaired electrons in the metal ion, (iii) the type of bonding that predominates in the compound.
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Question 4 View Details
A volatile liquid of unknown composition is to be analysed for its molar mass by the vapour‑density method. Procedure: 1. Accurately weigh 0.500 g of the liquid and place it in a 250 mL flask. 2. Heat the flask until the liquid vaporises and the vapour completely fills the flask. 3. The vapour is then collected over water at 25.0 °C. The volume of vapour collected is 180.0 mL. 4. Atmospheric pressure at the time of the experiment is 760 mmHg and the vapour pressure of water at 25.0 °C is 23.8 mmHg. Using the data above, answer the following questions.
Question Parts
(a)
Calculate the partial pressure of the vapour of the unknown liquid (in atm).
(b)
Determine the number of moles of vapour collected using the ideal‑gas equation (R = 0.0821 L·atm·mol⁻¹·K⁻¹).
(c)
Calculate the molar mass (g·mol⁻¹) of the unknown liquid.
(d)
Identify two possible sources of systematic error in this experiment and explain how each would affect the calculated molar mass.
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