waec model questions vol1 2024 chemistry | Practical

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Question 1 View Details
A discharge tube containing an unknown gaseous element is connected to a diffraction‑grating spectrometer. The grating has 5 000 lines cm⁻¹ (spacing d = 2.0 × 10⁻⁶ m). The first‑order (n = 1) diffraction angles for four bright lines are recorded as follows: | Line | Diffraction angle (°) | |------|-----------------------| | 1 | 11.8 | | 2 | 12.5 | | 3 | 14.1 | | 4 | 19.2 | Using the data, answer the questions that follow.
Question Parts
(a)
Calculate the wavelength (in nm) of each of the four observed lines. Show one complete calculation and give the remaining answers to the appropriate number of significant figures.
(b)
Identify the unknown element by comparing the calculated wavelengths with the Balmer series of hydrogen. State the element name.
(c)
For the most energetic transition (shortest wavelength), calculate the energy change ΔE in kJ mol⁻¹. Use h = 6.626 × 10⁻³⁴ J s, c = 3.00 × 10⁸ m s⁻¹ and NA = 6.022 × 10²³ mol⁻¹. Show the formula used.
(d)
State two possible sources of error that could affect the wavelength determination.
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Question 2 View Details
A 1.250 g sample of an unknown metallic element (M) is placed in a crucible, heated strongly, and completely converted to its oxide, MOₓ. After cooling, the mass of the oxide is 1.680 g. The metal is known to form a trivalent oxide of the type M₂O₃. Using the data, answer the following questions.
Question Parts
(a)
Calculate the atomic mass of the metal (M) in g mol⁻¹. Show all steps.
(b)
Using the periodic table, suggest a possible identity for the metal. State the group/period and justify your choice.
(c)
State one experimental source of error that could affect the calculated atomic mass.
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Question 3 View Details
A sample of an unknown metal oxide is analysed by reduction with hydrogen gas. The experimental data are as follows: - Mass of empty crucible: 0.500 g - Mass of crucible containing the oxide before reduction: 2.500 g - Mass of crucible containing the reduced metal after the reaction: 1.800 g Using the data provided, answer the questions below.
Question Parts
(a)
Calculate the mass of oxygen that was removed from the oxide during reduction.
(b)
Determine the number of moles of oxygen that were removed.
(c)
If the metal in the oxide is a single element, deduce the empirical formula of the oxide.
(d)
Suggest a possible metal that could form the oxide MO₂, using the atomic mass obtained in part (c). Give two plausible candidates.
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Question 4 View Details
A 0.1000 M solution of sodium hydroxide (NaOH) is used to titrate a sample of hydrochloric acid (HCl) solution. The following data are recorded: - Volume of HCl taken for the titration: 25.00 mL\n- Volume of NaOH required to reach the phenolphthalein endpoint: 31.45 mL\n- Molarity of NaOH: 0.1000 M\nUsing the information above, answer the questions that follow.
Question Parts
(a)
Calculate the number of moles of NaOH used in the titration.
(b)
From the stoichiometry of the reaction, determine the number of moles of HCl present in the 25.00 mL sample.
(c)
Calculate the concentration (M) of the HCl solution.
(d)
How many grams of HCl are present in 100.0 mL of this acid solution?
(e)
State two possible sources of error in this titration and briefly explain how each would affect the calculated concentration of HCl.
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