neco 2021 chemistry | Practical

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Question 1 View Details
A sample of an unknown element X was analysed by a mass‑spectrometer. The instrument gave the following peaks for the isotopes of X:\n\n| Isotope (mass number) | Relative abundance (%) |\n|-----------------------|------------------------|\n| X‑28 | 62.0 |\n| X‑30 | 38.0 |\n\nThe atomic number of X is 14. Using the data above, answer the following:
Question Parts
(a)
Calculate the relative atomic mass of element X to two decimal places.
(b)
Write the ground‑state electron configuration of a neutral atom of X using the Aufbau principle.
(c)
State the number of neutrons present in the most abundant isotope of X.
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Question 2 View Details
The following data were obtained from a series of measurements on the elements of the second period of the periodic table. The atomic radius (in pm) and first ionisation energy (IE, in kJ·mol⁻¹) are given:\n\n| Element | Atomic radius (pm) | First IE (kJ·mol⁻¹) |\n|---------|-------------------|----------------------|\n| Na | 186 | 496 |\n| Mg | 160 | 738 |\n| Al | 143 | 578 |\n| Si | 118 | 787 |\n| P | 110 | 1012 |\n| S | 104 | 1000 |\n| Cl | 99 | 1251 |\n| Ar | 71 | 1521 |\n\nUsing this information, answer the questions that follow:
Question Parts
(a)
Identify the element with the highest first ionisation energy and calculate the percentage increase in ionisation energy from Na to that element. Give your answer to one decimal place.
(b)
Assuming a linear decrease of atomic radius across the period, estimate the atomic radius of potassium (K), the first element of the next period, by extrapolating the trend from Na (186 pm) to Ar (71 pm). Round your answer to the nearest pm.
(c)
Provide two scientific explanations for the observed decrease in atomic radius from Na to Ar across the period.
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Question 3 View Details
You are given three white crystalline solids labelled X, Y and Z. The following experimental procedures were carried out on each solid: 1. A small amount of each solid was dissolved in 50.0 mL of distilled water and the solution was tested for electrical conductivity using a conductivity tester. 2. The solubility of each solid in water was observed (recorded as soluble or insoluble). 3. A clean nichrome wire was placed in the flame of a Bunsen burner and the colour of the flame was noted after a small amount of each solid was introduced into the flame. 4. A small piece of each solid was placed on a piece of moistened filter paper and the appearance of any reaction (e.g., effervescence) was noted. Record all observations and use them to answer the questions that follow.
Question Parts
(a)
Complete the observation table for X, Y and Z.
(b)
Using the observations, classify each compound as ionic, covalent (molecular) or metallic. Provide a brief justification for each classification.
(c)
Suggest a plausible empirical formula for each compound, assuming that any ionic compound contains a Group‑1 cation (Na⁺ or K⁺) and that the covalent compounds contain only non‑metals. State any assumptions made.
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Question 4 View Details
A volatile liquid of unknown composition is placed in a 250.0 mL eudiometer tube. The tube, when empty, weighs 45.0 g. After adding 3.6 g of the liquid, the tube is sealed and inverted in a water bath at 298 K. The gas released from the liquid occupies 112.0 mL at the top of the tube. Atmospheric pressure is 760 mmHg and the vapour pressure of water at 298 K is 23.8 mmHg. Using the data provided, answer the following questions.
Question Parts
(a)
Calculate the mass of the volatile liquid that actually vaporised.
(b)
Determine the number of moles of gas collected, correcting for the presence of water vapour.
(c)
Calculate the molar mass of the volatile liquid.
(d)
Identify two possible sources of systematic error in this experiment and suggest one method to minimise each error.
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