waec model questions vol1 2022 chemistry | Practical

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Question 1 View Details
A student is required to determine the relative atomic mass of magnesium by preparing magnesium oxide. The apparatus consists of a clean porcelain crucible with lid, a balance (to 0.01 g), a Bunsen burner and tongs. The procedure is: 1. Weigh the empty crucible (M₁). 2. Add a small piece of magnesium ribbon and weigh the crucible with magnesium (M₂). 3. Heat the crucible strongly until the magnesium has completely reacted with oxygen to form magnesium oxide. Allow to cool and weigh the crucible with the product (M₃). Given the following measurements: - Mass of empty crucible, M₁ = 45.00 g - Mass of crucible + magnesium, M₂ = 45.24 g - Mass of crucible + magnesium oxide, M₃ = 45.40 g (Atomic mass of oxygen = 16.0 g mol⁻¹) Answer the following:
Question Parts
(a)
Write the balanced chemical equation for the formation of magnesium oxide from magnesium and oxygen.
(b)
Calculate the mass of oxygen that combined with magnesium to form the oxide.
(c)
Determine the relative atomic mass of magnesium from the data obtained.
(d)
State one possible source of error that could affect the result and suggest how to minimise it.
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Question 2 View Details
An unknown metallic solid is to be identified using a flame test and its reaction with dilute hydrochloric acid. The following apparatus are provided: a clean nichrome wire loop, a Bunsen burner, a test tube containing dilute HCl, a test tube containing aqueous Na₂CO₃, and a set of labelled colour cards for flame colours. The procedure is: 1. Clean the wire loop and dip it into a small amount of the metal sample. 2. Place the loop in the flame of the Bunsen burner and observe the colour of the flame. 3. Record the observed flame colour using the colour card. 4. Place a small piece of the metal into a test tube containing dilute HCl and note the nature of the reaction (e.g., effervescence, colour change). 5. To the resulting solution add a few drops of Na₂CO₃ solution and note any precipitate formed. Observations recorded by the student are: - Flame colour: bright orange‑red. - Reaction with dilute HCl: vigorous effervescence with the evolution of a colourless gas. - After addition of Na₂CO₃: a white precipitate forms instantly. Using the observations, answer the following:
Question Parts
(a)
Identify the metal ion present in the unknown sample.
(b)
State the group and period of the identified element in the periodic table.
(c)
Explain, using periodic trends, why calcium gives the observed flame colour and reacts vigorously with dilute HCl compared with a metal from Group 13 such as aluminium.
(d)
Suggest one practical use of the identified metal and give a brief justification.
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Question 3 View Details
A student investigates the nature of bonding in three solid substances labelled A, B and C. The following experimental procedures are carried out: 1. A small quantity of each solid is dissolved in 50 mL of distilled water. 2. The conductivity of each solution is tested using a calibrated conductivity meter. 3. A small amount of each solid is heated over a Bunsen flame and the melting point is observed. 4. Each solid is placed in a clean porcelain crucible and heated strongly; the appearance of the residue is noted. 5. A flame test is performed by dipping a nichrome wire in the solid and introducing it into a non‑luminous Bunsen flame; the colour of the flame is recorded. The observations are recorded in the table below. | Substance | Conductivity of solution | Melting point (°C) | Residue on strong heating | Flame colour | |-----------|--------------------------|--------------------|---------------------------|--------------| | A | High | 770 | Metallic shiny | No colour | | B | Low (negligible) | 150 | White powder, no change | Yellow‑green | | C | Moderate | 660 | Brittle grey solid | No colour | Based on the above data, answer the questions that follow.
Question Parts
(a)
Identify the type of bonding present in each substance (A, B and C).
(b)
Substance B is found to be formed from a metal X (atomic mass 55.85 g mol⁻¹) and a non‑metal Y (atomic mass 35.45 g mol⁻¹). Write the empirical formula of the compound, assuming it is neutral.
(c)
Calculate the mass of metal X required to prepare 0.250 mol of the compound written in part (b).
(d)
Suggest one practical source of error in the conductivity test and state how it can be minimised.
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Question 4 View Details
A student carries out the reaction between sodium carbonate and hydrochloric acid to evolve carbon dioxide gas. The procedure is as follows: 1. Accurately weigh 2.00 g of solid Na₂CO₃ and transfer it into a 250 mL conical flask. 2. Add excess dilute HCl (0.1 M) to the flask and immediately fit a delivery tube leading to an inverted, water‑filled 500 mL graduated cylinder (the gas collection over water set‑up). 3. Record the volume of gas collected when the reaction is complete. The temperature of the water bath is 25 °C and the atmospheric pressure is 760 mm Hg. The vapour pressure of water at 25 °C is 23.8 mm Hg. 4. The theoretical stoichiometry for the reaction is: Na₂CO₃ + 2 HCl → 2 NaCl + H₂O + CO₂(g). The volume of gas collected is 450 mL. Using the data above, answer the following questions:
Question Parts
(a)
Calculate the number of moles of Na₂CO₃ that were initially weighed.
(b)
Determine the number of moles of CO₂ gas actually collected, correcting for the water vapour pressure.
(c)
Calculate the percent yield of CO₂ obtained in the experiment.
(d)
Identify two possible sources of error in this experiment and suggest how each could be reduced.
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