waec model questions vol1 2019 chemistry | Practical

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Question 1 View Details
A student performed a mass‑spectrometric analysis of a chlorine sample. The instrument gave the following peak heights: - Mass 35 u : 760 units - Mass 37 u : 240 units The detector is known to be 5 % less sensitive for ions of mass 37 u than for ions of mass 35 u. Using this information, answer the questions below.
Question Parts
(a)
Correct the measured intensities for the detector sensitivity and determine the relative abundance (as a fraction) of each isotope.
(b)
Using the abundances obtained in (a), calculate the average atomic mass of chlorine (in atomic mass units, u).
(c)
The accepted standard atomic mass of chlorine is 35.45 u. Calculate the percentage error of the experimental value obtained in (b).
(d)
Suggest two possible sources of error that could have contributed to the discrepancy between the experimental and standard atomic masses.
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Question 2 View Details
A student weighed a sample of an unknown metal carbonate (MCO₃) and recorded 2.00 g. The sample was heated in a crucible until a constant mass was obtained. The mass of the residue (metal oxide, MO) was 1.30 g. The decomposition reaction is: MCO₃(s) → MO(s) + CO₂(g) Using the data provided, answer the following questions.
Question Parts
(a)
Calculate the mass of CO₂ released during the heating.
(b)
Determine the number of moles of CO₂ evolved (M(CO₂) = 44.01 g mol⁻¹).
(c)
From the stoichiometry of the reaction, calculate the molar mass of the metal M (atomic mass, since the metal is monatomic).
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Question 3 View Details
A 0.500 g sample of an unknown solid electrolyte (X) was dissolved in distilled water to make two separate solutions of concentrations 0.010 mol dm⁻³ and 0.001 mol dm⁻³ respectively. The cell constant of the conductivity apparatus was calibrated as 0.50 cm⁻¹. The measured resistances (R) of the two solutions were: - 0.010 M solution: R = 200 Ω - 0.001 M solution: R = 1500 Ω Using the data provided, answer the following:
Question Parts
(a)
Calculate the specific conductivity (κ) of each solution in S cm⁻¹.
(b)
Calculate the molar conductivity (Λₘ) of each solution in S cm² mol⁻¹.
(c)
From the trend of Λₘ with dilution, state whether X behaves as a strong electrolyte or a weak electrolyte and, consequently, indicate the predominant type of chemical bond in X (ionic or covalent). Justify your answer in one sentence.
(d)
Suggest two possible sources of experimental error that could affect the determination of the electrolyte type.
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Question 4 View Details
A sample of a hydrated magnesium sulphate was analysed by gravimetric method. Approximately 1.23 g of the hydrate was placed in a crucible, heated gently to remove free water and then strongly until a constant mass was obtained. The mass of the anhydrous residue (MgSO₄) after cooling in a desiccator was recorded as 0.60 g. Using the data above, answer the following:
Question Parts
(a)
Calculate the mass of water that was lost on heating.
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