neco 2024 chemistry | Practical

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Question 1 View Details
A diffraction grating with 5000 lines cm\(^{-1}\) is used to observe the first line of the Balmer series of hydrogen (transition n = 3 \(\rightarrow\) n = 2). The angle between the incident beam and the first‑order diffracted beam is measured as 20.0°. (a) Calculate the wavelength of this Balmer line. (b) Using the Balmer formula \(\frac{1}{\lambda}=R\left(\frac{1}{2^{2}}-\frac{1}{n^{2}}\right)\) with \(n = 3\), determine the experimental value of the Rydberg constant \(R\). (c) Comment briefly on the accuracy of your experimental \(R\) value compared with the accepted value \(R = 1.097\times10^{7}\,\text{m}^{-1}\).
Question Parts
(a)
Calculate the wavelength of this Balmer line.
(b)
Using the Balmer formula with n = 3, determine the experimental Rydberg constant R.
(c)
Comment briefly on the accuracy of your experimental R value compared with the accepted value \(1.097\times10^{7}\,\text{m}^{-1}\).
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Question 2 View Details
A 1.20 g sample of an unknown metallic element (M) is placed in a sealed reaction tube and heated with excess chlorine gas at 25 °C. The reaction proceeds to completion forming a chloride of formula MCl\(_n\) (where \(n\) is an integer). After the reaction, the mass of the solid chloride obtained is 2.10 g. (a) Determine the value of \(n\) (the number of chlorine atoms combined with each metal atom). (b) Calculate the atomic mass of the metal M. (c) Using the periodic table, suggest a possible identity for the metal, giving its chemical symbol and atomic number.
Question Parts
(a)
Determine the value of n.
(b)
Calculate the atomic mass of the metal M.
(c)
Using the periodic table, suggest a possible identity for the metal, giving its chemical symbol and atomic number.
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Question 3 View Details
Three colourless crystalline solids labelled X, Y and Z are supplied for a chemistry practical. The following procedures are carried out for each sample:
Question Parts
(a)
A small quantity of each solid is added to 50 cm³ of distilled water, the mixture is stirred and the conductivity of the resulting solution is tested. Based on the conductivity observations, classify each compound as ionic, covalent or metallic.
(b)
Each solid is then melted in a crucible and the conductivity of the melt is tested. The observations are: X – high conductivity, Y – no conductivity, Z – low conductivity. Re‑evaluate the bonding classification of each sample using these melt‑conductivity results.
(c)
A flame test is performed on a small amount of each solid. The flame colours observed are: X – orange‑red, Y – no colour, Z – bright green. Explain the significance of these flame colours in relation to the type of bond present.
(d)
Summarise the overall bonding type for each sample (X, Y and Z) and justify your answer using the three experimental observations (aqueous conductivity, melt conductivity and flame test).
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Question 4 View Details
A student carries out an experiment to determine the amount of sodium carbonate in a solid sample using hydrochloric acid. The procedure is as follows:
Question Parts
(a)
The mass of the solid sample taken is 1.00 g. It is dissolved in water and excess 0.500 M HCl is added until the evolution of CO₂ stops. The volume of HCl used is recorded as 25.0 cm³. Calculate the number of moles of HCl that reacted.
(b)
The balanced equation for the reaction is: Na₂CO₃ + 2 HCl → 2 NaCl + CO₂ + H₂O. Using the moles of HCl calculated in part (a), determine: (i) the theoretical number of moles of Na₂CO₃ present in the sample, (ii) the theoretical mass of NaCl that should be produced.
(c)
After the reaction, the solution is evaporated to dryness and the solid NaCl obtained is weighed. The mass recorded is 0.58 g. Calculate the percent yield of NaCl based on the theoretical mass obtained in part (b)(ii).
(d)
Identify two possible sources of error that could affect the percent yield obtained and suggest a method to minimise each error.
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