A-LevelChemistryAnalytical techniques9701/52 · Oct/Nov 2025 · Q213 Marks

A student carries out an experiment to determine the concentration of aqueous sulfate ions, SO42-(aq), in a sample of lake water.

(a)(i)Suggest a suitable piece of apparatus to transfer 25.00 cm³ of the lake water sample to the beaker in step 1.[1]
(a)(ii)0.100 mol dm⁻³ Ba(OH)2(aq) is an irritant to skin and eyes. Other than wearing safety goggles, state one safety precaution that the student should take when conducting this experiment.[1]
(b)(i)Complete Table 2.1.[2]
(b)(ii)Identify the independent variable in this experiment.[1]
(c)Plot a graph on the grid in Fig. 2.2 to show the relationship between corrected conductivity and volume of 0.100 mol dm⁻³ Ba(OH)2 added to beaker. Use a cross (x) to plot each data point. Draw a line of best fit using readings 1 to 4 and another line of best fit using readings 5 to 9. Extend the lines so that they intersect.[2]
(d)(i)Use the graph in Fig. 2.2 to determine the volume of 0.100 mol dm⁻³ Ba(OH)2(aq) required to react exactly with SO42-(aq) in the lake water sample. volume required =[1]
(d)(ii)The equation for the reaction taking place in the beaker is shown. Ba2+(aq) + SO42-(aq) → BaSO4(s) Use your answer in (d)(i) to calculate the concentration of SO42-(aq) in the lake water sample. concentration of SO42-(aq) =[1]
(e)(i)Draw a labelled diagram to describe the arrangement of apparatus that would be needed to complete step 3.[1]
(e)(ii)Suggest a step that the student should add between steps 3 and 4 to improve this method.[1]
(e)(iii)Describe what the student can do to ensure that the residue weighed in step 5 is completely dry.[1]
(e)(iv)The mass of residue is used to calculate the concentration of SO42-(aq). Suggest the effect, if any, on the concentration of SO42-(aq) that is calculated if the residue is not completely dried in step 4. Explain your answer. effect on concentration of SO42-(aq) calculated explanation[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 (Oct/Nov 2025). The mark or mark type is shown in brackets.

[1] (25.0 cm³) volumetric pipette
[1] (wear) chemically resistant gloves
[2] M1 Total volume in beaker values
M2 Corrected conductivity values
[1] volume of 0.100 mol dm-³ Ba(OH)2(aq) added to beaker
[2] M1: all points 1–4 and 5–9 plotted correctly
M2: two straight lines of best fit line drawn going through most points. lines must intersect.
Place M2 tick in 'V' of graph
Place M1 tick or cross on left hand side of grid
Place cross (or highlight) against mis-plot
[1] intersection point volume (read from within grid)
[1] nSO42- =0.100 × ans (d)(i) / 1000 = mol [SO42-(aq)] = [0.100 × (d)(i) / 1000] / 0.025
[1] diagram showing labelled filter paper in a labelled (filter) funnel
[1] wash the residue with distilled water
[1] heat to constant mass
[1] higher value and (water would cause) (measured) mass / amount (of residue) would be greater