neco model questions vol1 2022 chemistry | Practical

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Question 1 View Details
A student carried out an experiment to determine the relative atomic mass of magnesium by reacting a weighed piece of magnesium ribbon with excess hydrochloric acid and collecting the hydrogen gas over water. Data supplied: - Mass of dry magnesium ribbon used: 0.500 g - Volume of gas collected over water: 620 mL - Temperature of the gas: 25 °C - Atmospheric pressure: 760 mmHg - Water‑vapour pressure at 25 °C: 23.8 mmHg - The reaction is: Mg (s) + 2 HCl (aq) → MgCl₂ (aq) + H₂ (g)
Question Parts
(a)
Calculate the number of moles of hydrogen gas collected, correcting for the presence of water vapour.
(b)
Using the stoichiometry of the reaction, calculate the experimental relative atomic mass of magnesium.
(c)
Identify two likely sources of error in this experiment and suggest one practical way to minimise each error.
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Question 2 View Details
A student performed a flame test on an unknown solid metal salt using a Bunsen burner and a handheld spectroscope. The dominant colour observed was a bright green and the spectroscope indicated that the most intense emission line had a wavelength of 520 nm. Reference data (wavelengths of the most intense flame‑colour line for selected metals): - Sodium (Na⁺): 589 nm (yellow) - Potassium (K⁺): 766 nm (violet‑lilac) - Calcium (Ca²⁺): 622 nm (brick‑red) - Barium (Ba²⁺): 553 nm (pale green) - Copper (Cu²⁺): 510 nm (bright green)
Question Parts
(a)
Calculate the energy, in joules, of a single photon corresponding to the observed wavelength of 520 nm. (Use h = 6.626 × 10⁻³⁴ J·s, c = 3.00 × 10⁸ m·s⁻¹.)
(b)
Identify the most likely metal ion present in the unknown sample by comparing the observed wavelength with the reference data provided.
(c)
Using the position of copper in the periodic table, explain why its flame colour is green, referring to the nature of the electronic transition responsible for the emission.
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Question 3 View Details
A student performed a series of qualitative tests on three unknown solid samples labelled A, B and C. The observations are recorded in the table below. | Sample | Solubility in water | Conductivity of the aqueous solution | Flame test colour | Additional observation | |--------|---------------------|--------------------------------------|-------------------|------------------------| | A | Soluble (colourless solution) | High (low resistance) | Lilac (characteristic of K⁺) | None | | B | Insoluble | No conduction | Bright white glow when heated in a Bunsen flame; the solid is malleable | Metallic luster | | C | Soluble (colourless solution) | Very low (practically non‑conducting) | No colour | Sweet taste (identifies as a sugar) | Based on the above data, answer the following questions. (a) Classify each sample as having an ionic, metallic or covalent (molecular) type of bonding. (b) Write a plausible chemical formula for each sample. (c) Explain the lilac flame colour observed for Sample A in terms of electron transitions. (d) State one practical application of each type of compound identified in part (a).
Question Parts
(a)
Classify each sample as having an ionic, metallic or covalent (molecular) type of bonding.
(b)
Write a plausible chemical formula for each sample.
(c)
Explain the lilac flame colour observed for Sample A in terms of electron transitions.
(d)
State one practical application of each type of compound identified in part (a).
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Question 4 View Details
In a laboratory experiment a clean, dry piece of magnesium ribbon was weighed and then reacted with excess hydrochloric acid. The hydrogen gas produced was collected over water. Data recorded: - Mass of magnesium before reaction: 0.150 g - Volume of gas collected: 155.0 mL - Temperature of the gas: 25 °C (298 K) - Atmospheric pressure: 760 mm Hg - Vapour pressure of water at 25 °C: 23.8 mm Hg The reaction is: Mg(s) + 2 HCl(aq) → MgCl₂(aq) + H₂(g) Using the data above, answer the following: (a) Calculate the number of moles of hydrogen gas collected. (b) Determine the experimental molar mass of magnesium from the data. (c) Calculate the percentage error of the experimental molar mass relative to the accepted value of 24.31 g mol⁻¹. (d) Identify two possible sources of error in the experiment and suggest how each could be minimised.
Question Parts
(a)
Calculate the number of moles of hydrogen gas collected.
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