Question 3
(a) Explain briefly how melting point determination could be used to distinguish between a pure sample and an impure sample. [2 marks]
(b) (i) What is dilution factor?
(ii) A concentrated acid has the following information on the bottle: Molar mass = 98 gmol-1,
density = 1.84 cm-3, % purity = 97. Calculate the concentration of the acid.
(iii) If 100 cm3 of the concentrated acid is diluted to 0.500 mol dm-3, calculate the dilution factor [8 marks]
(c) Asolid substance X when strongly heated, decomposes to give a white solid Y and carbon (IV) oxide. When water is added to Y, substance Z is produces.
i. Identify X, Y and Z.
ii. Suggest how Z could be used to test for carbon (IV) oxide indicating the observation to be made.
[5 marks]
(d) Consider the following gases:
NH3, H2, CO2, SO2
(i) State which of the gases is/are
(I) acidic;
(II) basic;
(III) neutral.
Observation
Majority of the candidates did not answer this question satisfactorily.
In part (a), majority of the candidates could not explain briefly how the melting point determination could be used to distinguish between a pure sample and an impure sample.
In part (b), majority of the candidates explained dilution factor. However, majority of them could not calculate the concentration of the acid from the given data.
In part (c), majority of the candidates were able to identify X, Y and Z.
In part (d), majority of the candidate were able to state which of the gases is/are acidic; basic and neutral. However, few of them were able to state the ones that could be collected by downward delivery.
The expected answers include:
(a) When the melting point is sharp (definite) / short or narrow range, then the sample/substance is pure
but when the melting point range is wide then the sample/substance is impure.
(b) 
OR
