waec model questions vol1 2020 chemistry | Practical

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Question 1 View Details
A student uses a magnetic sector mass spectrometer to analyse a pure metallic element X. The mass spectrum shows two isotope peaks at mass numbers 35 and 37. The recorded peak areas are 3.0 × 10⁵ for the 35‑amu peak and 1.0 × 10⁵ for the 37‑amu peak. The student also weighs a 0.500 g sample of the same metal on an analytical balance. Using the spectrometric data, the student is required to determine the relative atomic mass of element X and the amount of substance in the weighed sample.
Question Parts
(a)
Calculate the percentage abundance of each isotope of X from the peak‑area data.
(b)
Using the abundances from part (a), calculate the relative atomic mass of element X.
(c)
Determine the number of moles of X present in the 0.500 g sample using the relative atomic mass obtained in part (b).
(d)
Suggest one probable source of error that could affect the percentage abundances obtained from the mass‑spectrometer and explain how it would influence the calculated relative atomic mass.
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Question 2 View Details
A student carries out a flame‑test investigation to identify the metal ion present in an unknown aqueous sample (Sample U). The student prepares four standard solutions containing 0.10 M of Na⁺, K⁺, Ca²⁺ and Ba²⁺ respectively. A small amount of each solution is placed on a clean nichrome wire loop and introduced into a non‑luminous Bunsen‑burner flame. The observed flame colours are recorded. The same procedure is then performed with Sample U. In addition, the student adds a few drops of dilute HCl to a separate portion of Sample U and notes whether a precipitate forms.
Question Parts
(a)
State the flame colour observed for each of the four standard metal ions.
(b)
The flame of Sample U appears lilac and a white precipitate forms on addition of dilute HCl. Identify the metal ion(s) present in Sample U and give a brief justification using the observations and periodic‑table trends.
(c)
The lilac flame of potassium is due to an electronic transition that releases energy of 2.5 eV. Calculate the wavelength of the emitted radiation in nanometres (nm). (Use h = 6.626 × 10⁻³⁴ J·s, c = 3.00 × 10⁸ m s⁻¹, 1 eV = 1.602 × 10⁻¹⁹ J.)
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Question 3 View Details
A sample of an unknown metallic element is reacted with excess chlorine gas in a sealed vessel. The metal sample (mass = 1.20 g) is placed in the vessel and chlorine gas is introduced until no further change is observed. After the reaction, the mass of the resulting metal chloride is measured as 2.10 g. The atomic mass of the metal is known to be 24.3 g·mol⁻¹.
Question Parts
(a)
Calculate the mass of chlorine that combined with the metal.
(b)
Determine the number of moles of metal and of chlorine that reacted.
(c)
Using the mole values, deduce the empirical formula of the metal chloride.
(d)
State the type of chemical bond expected in this compound and justify your answer.
(e)
Suggest one probable source of experimental error that could affect the calculated formula.
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Question 4 View Details
A gas sample is collected over water at 298 K. The volume of the gas collected is 0.500 L and its mass is 0.845 g. The atmospheric pressure at the time of collection is 760 mmHg and the vapour pressure of water at 298 K is 23.8 mmHg.
Question Parts
(a)
Calculate the pressure (in atm) exerted by the dry gas.
(b)
Determine the number of moles of the dry gas using the ideal gas equation.
(c)
Calculate the molar mass of the gas (g·mol⁻¹).
(d)
Based on the calculated molar mass, suggest a possible identity for the gas from the list: CO₂, N₂O, CH₄, O₂. State the closest match.
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