Chemistry · Oct / Nov 2025 · Paper 62 · Question 2
0620
As printed in the paper
The question as text
A student investigates the reaction between acidic solution B and two different solutions of aqueous sodium hydroxide, solution C and solution D, using two different indicators. The student does two experiments. Experiment 1 • Rinse a burette with distilled water and then with solution C. • Fill the burette with solution C. Run some of solution C out of the burette so that the level of the solution is on the burette scale. • Record the initial burette reading. • Use a measuring cylinder to pour 25 cm3 of solution B into a conical flask. • Add five drops of methyl orange indicator and five drops of thymolphthalein indicator to the conical flask. • Stand the conical flask on a white tile. • Slowly add solution C from the burette to the conical flask, while swirling the flask, until the solution changes colour from red to orange. This is the first end-point. • Record the burette reading at the first end-point. • Continue to add solution C from the burette to the conical flask while swirling the flask. The solution changes colour from orange to yellow. • Continue to add solution C, while swirling the flask, until the solution changes colour from yellow to green. This is the second end-point. • Record the burette reading at the second end-point. Experiment 2 • Empty the conical flask and rinse it with distilled water. • Repeat Experiment 1 using solution D instead of solution C.
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2(a)[5]
M1 all burette readings correct in experiment 1 (21.2, 41.0, 1.6)
M2 all burette readings correct in experiment 2 (10.2, 20.1, 0.4)
M3 burette readings recorded in correct place in table
M4 all four titres in experiment 1 and 2 calculated correctly (19.6, 39.4, 9.8, 19.7)
M5 all ten readings/volumes given to 1 decimal place or better and a consistent number of decimal places
2(b)(i)[1]
to clean / to remove contaminants / to remove impurities
2(b)(ii)[2]
M1 (volume (added) is) greater / more / higher / bigger (if not rinsed with C)
M2 water left in the burette dilutes solution C / solution C becomes less concentrated
2(b)(iii)[1]
colour (changes) easier to see / colour change can be seen easily/clearly/better/accurately
2(c)[3]
M1 D / experiment 2 is more concentrated (than C/experiment 1)
OR
C / experiment 1 is less concentrated (than D/experiment 2)
M2 smaller volume of D (required to reach end point/neutralisation) OR larger volume C (required to reach end-point/neutralisation)
M3 answer is quantitative. Either D is 2 x concentration of C OR the volume of C is 2 x the volume of D
2(d)(i)[2]
M1 39.2
M2 units given as cm3
2(d)(ii)[1]
volume would be more than burette can hold
2(e)[2]
M1 none / no change / same volume needed
M2 (warming) does not change moles / concentration
2(f)[1]
use a volumetric pipette instead of a measuring cylinder
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