neco model questions vol1 2017 chemistry | Practical

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Question 1 View Details
A clean piece of an unknown metallic element was weighed accurately and then reacted with excess dilute hydrochloric acid in a closed eudiometer tube to liberate hydrogen gas. The experiment was carried out at 25 °C. The initial pressure of the system (air + water vapour) was 760 mm Hg. After the reaction, the volume of gas collected over water was 125 mL. The water‑vapour pressure at 25 °C is 23.8 mm Hg. The mass of the metal sample before the reaction was 0.512 g. (Assume the reaction proceeds according to M + 2HCl → MCl₂ + H₂).
Question Parts
(a)
Calculate the number of moles of hydrogen gas collected.
(b)
Using the data, determine the relative atomic mass of the metal.
(c)
State one possible source of error that could cause the calculated atomic mass to be higher than the true value.
(d)
Suggest one precaution to minimise the error mentioned in part (c).
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Question 2 View Details
A student is given an unknown aqueous solution of a metal ion. The following observations were recorded during the investigation: - Flame test: a bright blue‑green flame. - Addition of dilute NaOH solution produces a pale blue precipitate which dissolves on addition of excess NaOH, giving a deep blue solution. - Addition of aqueous ammonia to the solution after the NaOH test produces a deep blue complex that persists on standing. Using the periodic table and the observations, answer the questions below.
Question Parts
(a)
Identify the metal ion present in the solution.
(b)
Write the electron configuration of the neutral atom of this element.
(c)
State the group number and period of the element in the periodic table.
(d)
Explain, in terms of electronic transitions, why the flame test shows a blue‑green colour.
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Question 3 View Details
A student carried out a series of simple qualitative tests on three solid samples labelled A, B and C. The observations are recorded in the table below. | Sample | Solubility in water | Electrical conductivity (solid) | Electrical conductivity (solution/molten) | Approx. melting point | |--------|---------------------|--------------------------------|-------------------------------------------|-----------------------| | A | Dissolves readily | Does not conduct | Conducts well when dissolved | High (≈ 800 °C) | | B | Does not dissolve | Does not conduct | Does not conduct | Low (≈ 150 °C) | | C | Does not dissolve | Conducts well | Conducts well when molten | High (≈ 1200 °C) | Based on these observations answer the following questions:
Question Parts
(a)
Identify the predominant type of chemical bond present in each of the samples A, B and C.
(b)
For each sample, give a brief explanation (one or two sentences) linking the observed property to the type of bond you identified.
(c)
Sample D is an unknown solid. The observations for D are: (i) does not dissolve in water, (ii) does not conduct electricity in the solid state, (iii) has a high melting point (~1500 °C). State the most likely type of bonding in D and justify your choice.
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Question 4 View Details
A volatile liquid was examined by the eudiometer method to determine its molar mass. The following data were recorded: - Mass of empty eudiometer flask (dry) = 45.00 g - Mass of flask containing the liquid before heating = 46.20 g - Volume of gas collected after complete vaporisation = 0.850 L (measured at 298 K and 750 mmHg) - The gas collected is dry (no water vapour present). Using the ideal gas equation, answer the questions below:
Question Parts
(a)
Calculate the number of moles of gas collected.
(b)
Determine the experimental molar mass of the volatile liquid.
(c)
The accepted molar mass of the liquid is 28.0 g·mol⁻¹. Suggest two possible reasons for the discrepancy between the experimental and accepted values.
(d)
Explain why the ideal gas equation is applicable in this experiment.
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