waec model questions vol1 2023 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 the following relative ion‑current intensities for the two isotopic peaks observed. | Isotope | Relative intensity (arbitrary units) | |---------|--------------------------------------| | ³⁵Cl | 3000 | | ³⁷Cl | 1000 | Using this data, answer the questions below.
Question Parts
(a)
Calculate the relative atomic mass of chlorine from the data obtained.
(b)
The accepted natural isotopic abundances of ³⁵Cl and ³⁷Cl are 75.78 % and 24.22 % respectively. Discuss one possible source of error that could have caused the observed intensity ratio to differ from the natural abundance.
(c)
Suggest one procedural improvement to the experiment that could reduce the error identified in part (b).
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Question 2 View Details
A student performed a flame test and a series of qualitative reactions on an unknown solid metal sample. The observations recorded are as follows: | Test | Observation | |--------------------------|----------------------------------------------------------------------------------| | Flame test | Bright yellow flame with an intense orange component (characteristic of Na) | | Reaction with dilute HCl | Vigorous effervescence; colourless gas evolved | | Reaction with aqueous NaOH| White precipitate formed which dissolves on addition of excess NaOH (clear) | | Reaction with dilute H₂SO₄| No visible reaction | Answer the questions below.
Question Parts
(a)
Identify the metal ion present in the sample and state its group (or period) in the periodic table.
(b)
Write the electron configuration of the neutral metal atom.
(c)
Using periodic trends, explain (i) why the metal gives a yellow‑orange flame and (ii) why it reacts vigorously with dilute HCl to evolve a gas.
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Question 3 View Details
Three unknown solid compounds labelled X, Y and Z are subjected to the qualitative tests listed below. Observations: - Compound X: dissolves readily in water forming a clear solution; the solution conducts electricity strongly; on evaporation a white crystalline solid precipitates. - Compound Y: does not dissolve in water; does not conduct electricity; melts at about 150 °C with a faint odor. - Compound Z: a shiny metallic solid that does not dissolve in water but conducts electricity when a piece is placed between the terminals of a circuit. Using the observations, answer the questions that follow.
Question Parts
(a)
State the predominant type of chemical bonding in each of the compounds X, Y and Z.
(b)
Write a balanced chemical equation for the formation of compound X from its constituent elements in their standard states.
(c)
Suggest one source of experimental error that could lead to an incorrect conclusion about the bonding type of compound Y and propose a method to minimise it.
(d)
i) Explain why the conductivity test is a reliable indicator of ionic bonding. ii) Explain why metallic bonding exhibits electrical conductivity in the solid state.
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Question 4 View Details
A student carries out a gravimetric analysis to determine the purity of a sample of calcium carbonate (CaCO₃). The procedure is as follows: 1. A 1.250 g sample of the solid is weighed. 2. The sample is dissolved in excess hydrochloric acid. The carbon dioxide gas evolved is collected over water at 25 °C and 760 mmHg. The volume of gas collected is 0.560 L. The vapour pressure of water at 25 °C is 23.8 mmHg. 3. The calcium ion in solution is precipitated as calcium oxalate monohydrate by adding excess oxalic acid. The precipitate (CaC₂O₄·H₂O) is filtered, washed, dried and weighed, giving a mass of 0.450 g. Using the data above, answer the following questions.
Question Parts
(a)
Calculate the number of moles of CaCO₃ originally present in the sample using the gas‑collection data.
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