neco 2020 chemistry | Practical

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Question 1 View Details
A mass‑spectrometer was used to analyse an unknown gaseous element. The instrument gave two distinct peaks as follows: - Peak at mass number 35 with relative intensity 75 % - Peak at mass number 37 with relative intensity 25 % Using the data provided, answer the following:
Question Parts
(a)
Calculate the relative atomic mass of the element to two decimal places.
(b)
State the number of neutrons present in the most abundant isotope of the element.
(c)
Write the ground‑state electron configuration of the element using the Aufbau principle.
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Question 2 View Details
A student performed a flame test on an unknown inorganic salt dissolved in distilled water. The following observations were recorded: - The flame produced a bright yellow colour. - When the emission spectrum of the flame was examined with a spectroscope, a strong yellow line at a wavelength of 589 nm was noted. Answer the questions that follow.
Question Parts
(a)
Identify the metal ion present in the unknown salt.
(b)
State the period and group of this element in the periodic table.
(c)
Suggest a simple qualitative test that could be used to confirm the presence of this ion in the sample.
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Question 3 View Details
Three unknown solid samples labelled X, Y and Z are subjected to a series of qualitative tests in the chemistry laboratory. The observations obtained are recorded in the table below. | Test | Sample X | Sample Y | Sample Z | |------|----------|----------|----------| | (i) Solubility in 100 mL of water (2.0 g sample) | Completely soluble | Insoluble | Slightly soluble | | (ii) Conductivity of the resulting solution (if any) | Conducts electricity | No conductivity (dry sample) | Does not conduct electricity | | (iii) Melting point (°C) | 801 | 660 | 120 | | (iv) Reaction with dilute HCl (excess) | No visible reaction | Bubbles of gas evolve rapidly | No visible reaction | Using the above data, answer the following questions. (a) State the type of chemical bonding present in each of the three samples. (b) For each test (i)–(iv), explain how the observation helps to identify the type of bonding. (c) Suggest a possible chemical formula for each sample consistent with the identified bond type. (d) Propose one practical precaution that should be observed when performing test (iv).
Question Parts
(a)
State the type of chemical bonding present in each of the three samples.
(b)
For each test (i)–(iv), explain how the observation helps to identify the type of bonding.
(c)
Suggest a possible chemical formula for each sample consistent with the identified bond type.
(d)
Propose one practical precaution that should be observed when performing test (iv).
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Question 4 View Details
A student carries out a gravimetric analysis to determine the purity of a limestone sample (calcium carbonate). The procedure is as follows: 1. Accurately weigh 5.00 g of the powdered limestone and transfer it to a 250 mL conical flask. 2. Add excess dilute hydrochloric acid to react the calcium carbonate according to: \(\mathrm{CaCO_3 + 2HCl \rightarrow CaCl_2 + CO_2(g) + H_2O}\). 3. The carbon dioxide gas produced is collected over water at 25 °C. The volume of gas measured is 0.845 L. 4. The water vapour pressure at 25 °C is 23 mm Hg and the atmospheric pressure is 760 mm Hg. 5. The remaining solution is evaporated to dryness and the solid residue is heated strongly in a crucible to convert any calcium carbonate to calcium oxide: \(\mathrm{CaCO_3 \xrightarrow{\Delta} CaO + CO_2}\). The mass of the final CaO residue obtained is 0.702 g. Answer the following: a) Calculate the number of moles of \(\mathrm{CO_2}\) collected. b) Determine the mass of calcium carbonate originally present in the sample. c) Calculate the percent purity of the limestone sample with respect to calcium carbonate. d) Using the mass of CaO obtained, calculate the experimental percent yield of CaO based on the theoretical amount calculated from part (b). Comment on two possible sources of error that could account for the difference between the experimental and theoretical yields.
Question Parts
(a)
Calculate the number of moles of \(\mathrm{CO_2}\) collected.
(b)
Determine the mass of calcium carbonate originally present in the sample.
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