neco model questions vol1 2023 chemistry | Practical

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Question 1 View Details
A mass spectrometer was used to analyse a sample of an unknown element X. The instrument gave the following data for the two isotopes observed: - Isotope A: mass = 35.967 u, relative intensity = 3 parts - Isotope B: mass = 36.966 u, relative intensity = 2 parts The atomic number of element X is known to be 17. Answer the following:
Question Parts
(a)
Calculate the relative atomic mass of element X using the data above.
(b)
Determine the percent abundance of each isotope.
(c)
Write the nuclear notation for each isotope of element X.
(d)
State one possible source of error that could affect the measured intensity ratio in the mass spectrometer.
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Question 2 View Details
A flame‑test was performed on an unknown aqueous solution. The flame observed was bright yellow‑orange and a spectroscope showed a dominant emission line at a wavelength of 589 nm. Answer the following:
Question Parts
(a)
Identify the metal ion present in the unknown solution.
(b)
Calculate the energy of a single photon corresponding to the 589 nm emission (in joules) and the energy per mole of photons (in kJ mol⁻¹). Use h = 6.626 × 10⁻³⁴ J s and c = 3.00 × 10⁸ m s⁻¹; Avogadro's number = 6.022 × 10²³ mol⁻¹.
(c)
A second emission line at 400 nm is also known for the same element. Determine the difference in energy (kJ mol⁻¹) between the 400 nm photon and the 589 nm photon.
(d)
Briefly explain why the flame colour is produced in terms of electronic transitions of the metal atoms.
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Question 3 View Details
A student is investigating the nature of bonding in three solid copper compounds: (i) copper(II) sulfate pentahydrate, (ii) copper(II) chloride, and (iii) copper(II) nitrate. The student prepares 0.10 M aqueous solutions of each compound using distilled water, measures their electrical conductivity with a calibrated conductivity meter, and records the values shown in the table below. | Compound | Conductivity (mS cm⁻¹) | |----------|------------------------| | CuSO₄·5H₂O | 1.20 | | CuCl₂ | 1.48 | | Cu(NO₃)₂ | 1.35 | Using the data and additional observations, answer the following questions.
Question Parts
(a)
Classify each compound as ionic, covalent or non‑electrolyte based on the conductivity data and give a brief justification.
(b)
A magnetic susceptibility test on solid copper(II) nitrate shows it to be paramagnetic with a magnetic moment of 1.73 BM. Using this observation together with the conductivity result, deduce the type of bonding present in copper(II) nitrate and justify your answer.
(c)
Suggest one practical precaution that must be observed when preparing the solutions to avoid systematic error in the conductivity measurements, and explain why it is important.
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Question 4 View Details
A gravimetric analysis is carried out to determine the amount of calcium carbonate (CaCO₃) present in a sample of limestone. Procedure and data: 1. A 1.250 g portion of powdered limestone is accurately weighed. 2. The sample is dissolved in excess dilute HCl. 3. Excess ammonium oxalate solution is added, precipitating calcium oxalate, CaC₂O₄·H₂O. 4. The precipitate is filtered, washed with distilled water and dried at 110 °C to constant mass, giving 0.842 g of calcium oxalate. 5. The dried precipitate is then heated in a muffle furnace at 600 °C until conversion to calcium oxide (CaO) is complete. The final mass of CaO obtained is 0.456 g. Using the data above, answer the following questions:
Question Parts
(a)
Calculate the number of moles of calcium present in the original limestone sample.
(b)
Determine the percentage purity of CaCO₃ in the limestone sample.
(c)
Identify two possible sources of error that could cause the calculated % purity to be lower than the true value, and suggest one way to minimise each error.
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