neco model questions vol1 2018 chemistry | Practical

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Question 1 View Details
A spectroscope is used to observe the Balmer series lines of hydrogen. The wavelengths recorded are: Hα (n=3→2) = 656.3 nm, Hβ (n=4→2) = 486.1 nm, Hγ (n=5→2) = 434.0 nm.
Question Parts
(a)
Calculate the Rydberg constant (R) for each of the three observed lines using the relation \(\frac{1}{\lambda}=R\left(\frac{1}{2^{2}}-\frac{1}{n^{2}}\right)\).
(b)
Determine the average value of the Rydberg constant from the three results obtained in part (a).
(c)
Using the average Rydberg constant, calculate the wavelength of the transition from n = 6 to n = 2 (Balmer series).
(d)
Comment briefly on the reliability of the average Rydberg constant obtained.
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Question 2 View Details
A student determines the atomic weight of magnesium by oxidising a known mass of magnesium ribbon in a crucible. The masses recorded are: Empty crucible: 15.000 g Crucible + Mg ribbon (before heating): 15.500 g Crucible + MgO (after heating to constant mass): 15.830 g
Question Parts
(a)
Calculate the mass of magnesium, the mass of oxygen that combined with it, and the atomic weight of magnesium (assume Mg forms MgO with valency 2).
(b)
State two possible sources of error that could affect the result obtained in part (a).
(c)
Suggest one precaution that can be taken to minimise the error mentioned in part (b).
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Question 3 View Details
A white crystalline solid of unknown composition is to be investigated for the type of chemical bonding present. The solid is tested for (i) electrical conductivity in the solid state, (ii) solubility in water, and (iii) electrical conductivity of its aqueous solution. Based on the observations, the nature of the bonding (ionic, covalent or metallic) should be deduced.
Question Parts
(a)
State the procedure for testing the electrical conductivity of the solid sample.
(b)
State the procedure for testing the solubility of the solid in water and the conductivity of the resulting solution.
(c)
Record the observations for the three tests and, using the data, deduce the type of bonding in the solid. State the reasoning for your conclusion.
(d)
Suggest two possible sources of experimental error in the above investigations and recommend how each could be minimised.
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Question 4 View Details
An unknown metal is to be identified by determining its molar mass using the reaction with dilute hydrochloric acid. The metal is reacted with excess HCl, the hydrogen gas evolved is collected over water and its volume measured. The data obtained are to be used to calculate the molar mass of the metal and to suggest its identity.
Question Parts
(a)
Write the balanced chemical equation for the reaction between the metal (M) and hydrochloric acid.
(b)
Outline the experimental set‑up and the steps required to collect the hydrogen gas over water and to obtain the necessary data for calculations.
(c)
The mass of the metal used was 0.500 g. The volume of hydrogen gas collected at 25 °C was 520 mL. Atmospheric pressure was 760 mmHg and the water‑vapour pressure at this temperature is 23.8 mmHg. Calculate the number of moles of hydrogen gas collected. (Use R = 0.0821 L·atm·mol⁻¹·K⁻¹)
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