A student carries out an investigation into the reaction between dilute hydrochloric acid and aqueous sodium hydroxide.
The student carries out five experiments.
Experiment 1
• Put aqueous sodium hydroxide into a burette.
• Let some aqueous sodium hydroxide flow out of the burette until its level is on the burette scale.
• Note the starting burette reading.
• Use a volumetric pipette to place 25.0 cm$^3$ of dilute hydrochloric acid into a conical flask.
• Place the conical flask on a white tile.
• Add five drops of thymolphthalein indicator to the conical flask.
• Gently add aqueous sodium hydroxide from the burette to the conical flask while swirling the flask, until the solution only just changes colour. This is the end-point.
• Note the finishing burette reading.
Experiment 2
• Discard the contents of the conical flask and wash it with distilled water.
• Refill the burette with aqueous sodium hydroxide.
• Let some aqueous sodium hydroxide flow out of the burette until its level is on the burette scale.
• Note the starting burette reading.
• Use the volumetric pipette to place 25.0 cm$^3$ of dilute hydrochloric acid into the conical flask.
• Add 0.25 g of sodium hydrogencarbonate to the conical flask and swirl the flask.
• Place the conical flask on the white tile.
• Add five drops of thymolphthalein indicator to the conical flask.
• Gently add aqueous sodium hydroxide from the burette to the conical flask while swirling the flask, until the solution only just changes colour. This is the end-point.
• Note the finishing burette reading.
Experiment 3
• Carry out Experiment 2 again, but use 0.50 g of sodium hydrogencarbonate instead of 0.25 g.
Experiment 4
• Carry out Experiment 2 again, but use 1.00 g of sodium hydrogencarbonate instead of 0.25 g.
Experiment 5
• Carry out Experiment 2 again, but use 1.50 g of sodium hydrogencarbonate instead of 0.25 g.
(a)[5]
Use the details in the descriptions of Experiments 1 to 5 together with the burette diagrams Fig. 2.1 and Fig. 2.2 to complete Table 2.1.
(b)[4]
Complete a suitable scale on the y-axis and plot the results from Experiments 1 to 5 in Table 2.1 on Fig. 2.3. Draw a straight line of best fit.
(c)[3]
Extend the line on your graph in Fig. 2.3 and deduce the mass of sodium hydrogencarbonate needed to neutralise all of the dilute hydrochloric acid in the conical flask. Make it clear on Fig. 2.3 how you obtained your answer.
(d(i))[1]
Explain why a volumetric pipette is used rather than a measuring cylinder to measure the volume of dilute hydrochloric acid used in each experiment.
(d(ii))[1]
Explain why it is not possible to use a volumetric pipette instead of the burette to measure the volume of aqueous sodium hydroxide added in each experiment.
(e)[2]
Explain why universal indicator is not a suitable indicator for this titration.
(f)[2]
Draw a line on Fig. 2.3 to show the results you would expect if the investigation was repeated using aqueous sodium hydroxide with twice the concentration. Label your line F.
Worked solution & mark scheme
This 18-mark question has a full step-by-step worked solution and mark scheme. One marking point: “M1 all five experiments have volume of hydrochloric acid recorded as 25.0 (cm^{3})” …