A-LevelChemistryAnalytical techniques9701/52 · Feb/Mar 2019 · Q210 Marks

A student was given a sample of an unknown Group 2 chloride. The student dissolved 3.172g of the unknown Group 2 chloride in distilled water in a beaker and added an excess of aqueous silver nitrate, AgNO3(aq), to the beaker. A white precipitate of silver chloride formed.

(a)Write the ionic equation, including state symbols, for the reaction occurring. [1][1]
(b)To separate the filtrate from the residue, filtration can be carried out using gravity or by using reduced pressure. The student decided to filter the mixture under reduced pressure.
(b)(i)Complete the labelled diagram to suggest how the student could filter the mixture under reduced pressure. [Figure showing a Buchner flask connected to a suction pump, with a partial funnel setup.] [2][2]
(b)(ii)Suggest one major advantage of filtering the mixture under reduced pressure compared with filtering using gravity. [1][1]
(c)The student rinsed the residue, transferred it to a crucible and placed it in a warm oven to dry it.
(c)(i)What should the student do to ensure that the drying process is complete? [1][1]
(c)(ii)The student recorded the masses shown in the table. Complete the table to calculate the mass of dry silver chloride formed. Use this value to determine the number of moles of silver chloride formed. [A: Cl, 35.5; Ag, 107.9] [Table with mass of crucible + dry silver chloride/g (24.898) and mass of empty crucible/g (19.162)] moles of silver chloride formed = mol [1][1]
(c)(iii)Use your answer to (ii) to calculate the mass of one mole of the Group 2 chloride and hence identify the Group 2 metal present in the chloride. If you were unable to calculate an answer in (ii), assume that 0.0304 mol of silver chloride formed. This is not the correct value. [Ar: Be, 9.0; Mg, 24.3; Ca, 40.1; Sr, 87.6; Ba, 137.3] mass of one mole of the Group 2 chloride = g identity of the Group 2 metal = [3][3]
(c)(iv)State and explain how the number of moles of silver chloride formed in (ii) would change if the student used tap water instead of distilled water to dissolve the Group 2 chloride. [1][1]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2019). The mark or mark type is shown in brackets.

[1] Ag+(aq) + Cl-(aq) → AgCl(s)
[1] (buchner) funnel (with filter paper)
[1] sealed with a cork around the funnel
[1] increased rate (of filtration)
[1] warm / heat to constant mass
[1] (5.736 g AND 5.736 / 143.4 =) 0.04(00)
[1] mol of MCl2 = 0.04 / 2 = 0.02(00)
[1] molar mass of MCl2 = 3.172/0.02 = 158.6
[1] Ar of M = 158.6 – 71.0 = 87.6 and Sr
[1] increase because tap water contains chloride ions