Chemistry · Oct / Nov 2025 · Paper 61 · Question 2
0620
As printed in the paper
The question as text
A student investigates the reaction between dilute hydrochloric acid and aqueous sodium hydroxide. The student does five experiments. Experiment 1 • Fill a burette with aqueous sodium hydroxide. • Run some of the aqueous sodium hydroxide out of the burette so that the level of the aqueous sodium hydroxide is on the burette scale. • Record the initial burette reading. • Use a volumetric pipette to add 25.0 cm3 of dilute hydrochloric acid to a conical flask. • Stand the conical flask on a white tile. • Add five drops of thymolphthalein indicator to the conical flask. • Slowly add aqueous sodium hydroxide from the burette to the conical flask, while swirling the flask, until the solution just changes colour. This is the end-point. • Record the final burette reading. Experiment 2 • Empty the conical flask and rinse it with distilled water. • Refill the burette with aqueous sodium hydroxide. • Run some of the aqueous sodium hydroxide out of the burette so that the level of the aqueous sodium hydroxide is on the burette scale. • Record the initial burette reading. • Use the volumetric pipette to add 25.0 cm3 of dilute hydrochloric acid to the conical flask. • Add 0.25 g of sodium hydrogencarbonate to the conical flask and swirl the flask. • Stand the conical flask on the white tile. • Add five drops of thymolphthalein indicator to the conical flask. • Slowly add aqueous sodium hydroxide from the burette to the conical flask, while swirling the flask, until the solution just changes colour. This is the end-point. • Record the final burette reading. Experiment 3 • Repeat Experiment 2 using 0.50 g of sodium hydrogencarbonate instead of 0.25 g. Experiment 4 • Repeat Experiment 2 using 1.00 g of sodium hydrogencarbonate instead of 0.25 g. Experiment 5 • Repeat Experiment 2 using 1.50 g of sodium hydrogencarbonate instead of 0.25 g.
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Show the answer and mark scheme
2(a)[5]
M1 all five experiments have volume of hydrochloric acid recorded as 25.0 (cm3)
M2 final and initial burette readings for experiment 1 correct and recorded the correct way round (46.1, 0.6)
M3 final and initial burette readings for experiment 2 correct and recorded the correct way round (41.3, 1.3)
M4 all titres calculated correctly (45.5, 40.0, 34.6, 23.8)
M5 all readings recorded to 1 decimal place
2(b)[4]
M1 y-axis scale is linear, and the points extend over halfway up the scale
M2 AND M3 all points plotted correctly
M4 best-fit line correctly drawn
2(c)[3]
M1 line is extrapolated to reach x-axis (to the right)
M2 correct reading from graph at volume of sodium hydroxide = 0.0 cm3
M3 units of g
2(d)(i)[1]
(volumetric pipette is) more accurate (than a measuring cylinder)
2(d)(ii)[1]
volumetric pipettes measure a fixed volume
2(e)[2]
M1 no sharp colour change / keeps changing colour / many colours / more than two colours / gradual colour change / sharp colour change is needed
M2 so cannot determine end-point / end-point not clear / difficult to tell when to stop adding /
2(f)[2]
M1 sketch line is below plotted line at all volumes
M2 line is straight, starts on y-axis between 20 and 25 and ends on x-axis between 2.0 and 2.2
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