neco model questions vol1 2020 chemistry | Practical

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Question 1 View Details
A student is given an unknown metallic salt solution. She performs a flame test using a Bunsen burner and a clean platinum‑wire loop. The flame observed is a bright crimson red.
Question Parts
(a)
State the procedure for performing the flame test, mentioning at least three precautions that must be observed.
(b)
Record the colour of the flame observed.
(c)
Identify the metal ion most likely present in the sample, using the standard flame‑colour chart supplied in the laboratory manual.
(d)
Explain, in terms of electron transitions, why the observed colour is produced.
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Question 2 View Details
A student carries out a gravimetric analysis to determine the percentage composition of copper in a sample of an unknown copper(II) oxide.
Question Parts
(a)
Write the balanced chemical equation for the formation of copper(II) oxide when copper metal is heated in air.
(b)
Using the data below, calculate (i) the mass of CuO obtained after heating, (ii) the mass of copper present in the CuO, and (iii) the percentage by mass of copper in the oxide. Data: mass of empty crucible = 45.12 g; mass of crucible + CuO after heating = 45.78 g.
(c)
From the percentage obtained, identify the unknown oxide by comparing with the theoretical percentages given below: CuO – 79.9 %; Fe₂O₃ – 69.9 %; ZnO – 80.6 %; Al₂O₃ – 53.1 %.
(d)
Discuss two possible sources of error in this gravimetric determination and explain how each could affect the calculated percentage of copper.
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Question 3 View Details
A 0.500 g sample of an unknown metal chloride is dissolved in distilled water. Excess aqueous silver nitrate solution is added, causing the chloride ions to precipitate as silver chloride. The precipitate is filtered, washed, dried and weighed. The mass of the dried AgCl precipitate is 0.345 g. The molar mass of AgCl is 143.32 g·mol⁻¹ and the atomic mass of chlorine is 35.45 g·mol⁻¹.
Question Parts
(a)
Calculate the mass of chloride ion present in the original sample.
(b)
Determine the empirical formula of the metal chloride.
(c)
State the type of chemical bond present in the compound and justify your answer based on the experimental findings.
(d)
Suggest two practical precautions that should be observed to minimise error in the precipitation method used above.
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Question 4 View Details
A 0.250 g sample of an unknown monoprotic solid acid (theoretical molar mass = 120 g·mol⁻¹) is dissolved in 250.0 mL of distilled water. The solution is titrated with 0.1000 M NaOH using phenolphthalein as the indicator. The endpoint is reached after 25.00 mL of the NaOH solution has been added.
Question Parts
(a)
Write the balanced chemical equation for the neutralisation reaction between the unknown acid (HA) and NaOH.
(b)
Calculate the number of moles of NaOH used and hence the number of moles of the unknown acid that reacted.
(c)
Determine the experimental molar mass of the unknown acid.
(d)
Calculate the percent purity of the sample, assuming the only impurity is inert.
(e)
Identify two possible sources of error in this titration and suggest a method to minimise each error.
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