waec model questions vol1 2021 chemistry | Practical

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Question 1 View Details
A clean dry piece of magnesium ribbon is weighed and then reacted with excess dilute hydrochloric acid in a conical flask. The hydrogen gas produced is collected over water. The data obtained are: - Mass of magnesium ribbon before the reaction: 0.150 g - Volume of hydrogen collected: 158.0 mL - Temperature of the gas: 298 K - Atmospheric pressure: 100 kPa - Vapour pressure of water at 298 K: 3.2 kPa Using the ideal‑gas equation, determine the relative atomic mass of magnesium. State the balanced chemical equation and discuss two possible sources of experimental error and how they would affect the result.
Question Parts
(a)
Write the balanced chemical equation for the reaction between magnesium and hydrochloric acid.
(b)
Calculate the number of moles of hydrogen gas collected, correcting for the vapour pressure of water.
(c)
From the stoichiometry of the reaction, determine the number of moles of magnesium that reacted.
(d)
Using the initial mass of magnesium, calculate its relative atomic mass.
(e)
State two possible sources of error in this experiment and explain qualitatively how each would affect the calculated atomic mass of magnesium.
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Question 2 View Details
Three unknown solid metal chlorides are labelled Sample A, Sample B and Sample C. Each sample is introduced separately into a clean Bunsen‑burner flame and the colour of the flame is observed. The observed colours are: - Sample A: bright yellow - Sample B: crimson red - Sample C: lilac Using the periodic table, identify the most probable metal ion present in each sample, write the ground‑state electron configuration of the neutral atom of that metal, and briefly explain why the flame emits the observed colour.
Question Parts
(a)
Record the flame colour observed for each sample.
(b)
Using characteristic flame colours of Group 1 and Group 2 metals, suggest the most likely metal ion for each sample (provide the element symbol).
(c)
Write the ground‑state electron configuration of the neutral atom of each identified metal (use the noble‑gas shorthand).
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Question 3 View Details
A student performed a series of qualitative tests on an unknown white crystalline solid labelled X. The observations are recorded in the table below. | Test | Observation | |------------------------------------|-----------------------------------------------------------| | Solubility in water | Dissolves with effervescence, forming a clear solution. | | Solubility in ethanol | Does not dissolve. | | Solubility in dilute HCl (0.1 M) | No change. | | Conductivity of 1 dm³ solution | High conductivity. | | Flame test | No colour observed. | | Reaction with AgNO₃ (2 mL) | White precipitate forms immediately. | A separate experiment was carried out by reacting 0.50 g of X with excess AgNO₃ solution. After filtration, 1.23 g of white precipitate (AgCl) was obtained. Answer the following questions:
Question Parts
(a)
Identify the anion present in X based on the observations.
(b)
Identify the cation present in X.
(c)
Using the data that 0.50 g of X gave 1.23 g of AgCl precipitate, calculate:
() Moles of AgCl formed.
() Percentage by mass of chlorine in X.
(d)
Write the empirical formula of X and briefly justify your answer.
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Question 4 View Details
A student wishes to determine the molar mass of an unknown monoprotic acid (HX) by standardisation against a sodium hydroxide solution. Procedure: 1. Accurately weigh 0.300 g of the solid acid and dissolve it in distilled water. 2. Add a few drops of phenolphthalein indicator. 3. Titrate the solution with 0.1000 M NaOH until a faint pink colour persists for 30 s. 4. The titration is repeated three times and the average volume of NaOH required to reach the endpoint is 24.60 mL. Answer the following questions:
Question Parts
(a)
Calculate the number of moles of NaOH used in the titration.
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