A student studies how acidic solution B reacts with two different aqueous sodium hydroxide solutions, solution C and solution D, by using two indicators.
The student carries out two experiments.
Experiment 1
• Rinse a burette first with distilled water and then with solution C.
• Fill the burette with solution C. Let some of solution C run out so that the liquid level is on the burette scale.
• Note the initial burette reading.
• Use a measuring cylinder to pour 25 cm$^3$ of solution B into a conical flask.
• Add five drops of methyl orange indicator and five drops of thymolphthalein indicator to the conical flask.
• Put the conical flask on a white tile.
• Slowly add solution C from the burette to the conical flask, swirling the flask, until the solution changes colour from red to orange. This is the first end-point.
• Note the burette reading at the first end-point.
• Keep adding solution C from the burette to the conical flask while swirling the flask. The solution changes colour from orange to yellow.
• Continue adding solution C, while swirling the flask, until the solution changes colour from yellow to green. This is the second end-point.
• Note the burette reading at the second end-point.
Experiment 2
• Empty the conical flask and rinse it with distilled water.
• Repeat Experiment 1 with solution D instead of solution C.
(a)[5]
Use the burette diagrams in Fig. 2.1 and Fig. 2.2 to enter the readings for Experiment 1 and Experiment 2 in Table 2.1. Finish Table 2.1.
(b(i))[1]
Explain why the conical flask is rinsed with distilled water at the start of Experiment 2.
(b(ii))[2]
Explain how the volume added from the burette to reach the first end-point would change if the burette was not rinsed with solution C.
(b(iii))[1]
Explain why the conical flask is stood on a white tile during the titration.
(c)[3]
Compare the concentration of solution C used in Experiment 1 with that of solution D used in Experiment 2. Explain your answer.
(d(i))[2]
Deduce the volume of solution C needed to reach the first end-point if Experiment 1 is repeated with 50 cm$^3$ of solution B rather than 25 cm$^3$.
(d(ii))[1]
State why using 50 cm$^3$ of solution B would cause a problem with finding the volume of solution C needed to reach the second end-point in Experiment 1.
(e)[2]
State the effect, if any, on the volume of solution D required to reach the second end-point in Experiment 2. Explain your answer.
(f)[1]
State one apparatus change that would improve the accuracy of the results.
Worked solution & mark scheme
This 18-mark question has a full step-by-step worked solution and mark scheme. One marking point: “every burette reading for experiment 1 is correct (21.2, 41.0, 1.6)” …