Question 119 marks
Question
You are going to investigate the reaction between acidic solution B and two different solutions of aqueous sodium hydroxide, solution C and solution D, using two different indicators. Read all of the instructions carefully before starting the experiments. Instructions You are going to do two experiments. Experiment 1 • Rinse the 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 between the 0.0 cm3 and 1.0 cm3 mark. • Record the initial burette reading in Table 1.1. • Use the measuring cylinder to pour 25 cm3 of solution B into the 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 in Table 1.1. • 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 in Table 1.1. Experiment 2 • Empty the conical flask and rinse it with distilled water. • Repeat Experiment 1 using solution D instead of solution C. (a) Complete Table 1.1. Table 1.1 Experiment 1 Experiment 2 using solution C using solution D burette reading at first end-point / cm3 burette reading at second end-point / cm3 initial burette reading / cm3 volume added from burette to reach first end-point / cm3 total volume added from burette to reach second end-point / cm3 [6] * 0000800000003 * DFD 3 (b) (i) Explain why the conical flask is rinsed with distilled water at the start of Experiment 2. …… [1] (ii) At the start of Experiment 1, the burette is rinsed with distilled water and then with solution C. Explain how the volume added from the burette to reach the first end-point would be different if the burette was not rinsed with solution C. …… [2] (iii) Explain why the conical flask is placed on a white tile during the titration. …… [1] (c) Compare the concentration of solution C used in Experiment 1 with the concentration of solution D used in Experiment 2. Explain your answer. …… [3] (d) (i) Deduce the volume of solution C required to reach the first end-point if Experiment 1 is repeated using 50 cm3 of solution B instead of 25 cm3. volume of solution C = …… [2] (ii) State why using 50 cm3 of solution B would cause a problem when finding the volume of solution C needed to reach the second end-point in Experiment 1. …… [1] * 0000800000004 * DFD 4 (e) A student repeats Experiment 2. The student warms solution B in the conical flask before carrying out the titration. State the effect, if any, on the volume of solution D required to reach the second end-point in Experiment 2. Explain your answer. effect on volume of solution D …… explanation …… [2] (f) State one change to the apparatus that will improve the accuracy of the results. …… [1] [Total: 19] * 0000800000005 * DFD 5
Mark scheme
1(a) M1 experimental results fully completed with initial burette readings< readings at first end =point < readings at second end 1 point M2 both first end-point titres correct 1 M3 both second end-point titres correct 1 M4 in experiment 1 the volume needed for second end-point is approximately 2 x the volume needed for first end-point 1 M5 in experiment 2 the first end-point is half experiment 1 first end-point 1 M6 all readings/volumes given to 1 decimal place or better and a consistent number of decimal places used 1 1(b)(i) to clean / to remove contaminants / to remove impurities 1 1(b)(ii) M1 volume (added) is greater / more / higher / bigger (if not rinsed with C) 1 M2 water left in the burette dilutes solution C / solution C becomes less concentrated 1 1(b)(iii) colour (changes) easier to see / colour change can be seen easily / clearly / better / accurately 1 1(c) M1 D / experiment 2 is more concentrated (than C / experiment 1) 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 1 end-point / neutralisation) M3 answer is quantitative. Either D is 2 × concentration of C OR the volume of C is 2 × the volume of D 1 1(d)(i) M1 correct evaluation of 2 × first end-point volume for experiment 1 1 M2 units given as cm3 1 1(d)(ii) volume would be more than burette can hold 1 1(e) M1 none / no change / same volume needed 1 M2 (warming) does not change moles / concentration 1 1(f) use a volumetric pipette instead of a measuring cylinder 1