1) Irfan is conducting an experiment in which two aqueous solutions, hydrogen peroxide, H₂O₂, and a vitamin C solution are reacted together in a beaker. This reaction is known to have a delayed but obvious colour change. The vitamin C solution is prepared by crushing a vitamin C tablet and dissolving it in deionised water, then adding iodine, I₂. Irfan is investigating how changing the concentration of vitamin C in the solution affects the time taken for the colour change to occur. The concentration is changed by adding different volumes of deionised water to 5.0 mL of the vitamin C solution before reacting it with H₂O₂. Irfan repeats the experiment twice for each concentration.

a. Some of the equipment used during the experiment is shown below.

The resolution of the equipment shown, from lowest to highest, is
A. 3 mL graduated plastic pipette < 10 mL measuring cylinder < 50 mL measuring cylinder < 50 mL beaker
B. 10 mL measuring cylinder < 3 mL graduated plastic pipette < 50 mL measuring cylinder < 50 mL beaker
C. 50 mL beaker < 50 mL measuring cylinder < 3 mL graduated plastic pipette < 10 mL measuring cylinder
D. 50 mL beaker < 50 mL measuring cylinder < 10 mL measuring cylinder < 3 mL graduated plastic pipette

Solution

b. A graph showing the time for a colour change to occur versus the amount of deionised water added is shown below.

Which one of the following factors is most likely responsible for the variability shown in the graph above?
A. judging when the colour change occurs
B. volume of iodine remaining in the tip of the plastic pipette
C. some of the vitamin C tablet not completely dissolving
D. changes in the laboratory room temperature during the experiment

Solution

2. Many vegetables contain compounds that taste bitter because of the oxalate ion, C₂O₄^2- . Removing the bitterness can be achieved through different cooking methods, including boiling and steaming. VCE Chemistry student Karolina undertook research to collect background information for her scientific investigation and learnt that oxalate compounds can be water-soluble and water-insoluble. From her research Karolina developed the following hypothesis: ‘The boiling process will remove 75% of the oxalate compounds in spinach leaves.’ Karolina tested her hypothesis by investigating the effectiveness of boiling to remove C₂O₄^2- from spinach leaves. The method developed by Karolina is outlined below.

a. Circle the scientific investigation methodology that Karolina most likely used to develop her hypothesis. (1 mark)

case study   fieldwork   literature review  modelling

C₂O₄^2- reacts with permanganate ions, MnO₄^-, according to the equation below

2MnO₄^- (aq) + 5C₂O₄^2-(aq) + 16H⁺(aq) → 2Mn²⁺(aq) + 10CO₂(g) + 8H₂O(l)

b. Refer to item 7 of the Data Book. What specific observation did Karolina use to determine the end point of the titration? (1 mark)

Solution

c. The reaction was conducted at 50 °C rather than at room temperature.

i. Use collision theory to state why this is a preferred temperature at which to

conduct the reaction. (1 mark)

ii. State why the experiment might no longer be valid if the reaction is conducted at room temperature. (1 mark)

Solution

Results Karolina recorded the titration results for the uncooked spinach in the table below.

d. Calculate the average titre of MnO₄^- used in the experiment. (1 mark)

Solution

e. A 15.15 g sample of uncooked spinach was used. Calculate the mass of C₂O₄^2-(M = 88.0 g mol^-1) in the sample using the experimental results above. Give your answer in grams. (4 marks)

Solution

Karolina then prepared a second 15.15 g sample of spinach as follows:
• boiled in deionised water for two minutes
• thoroughly rinsed with deionised water using a sieve
• left overnight to dry
This cooked sample was then tested using the steps outlined in Part A and Part B above.

f. Karolina calculated the mass of C₂O₄^2- to be 0.0545 g in this sample. Determine the percentage change of C₂O₄^2- that resulted by boiling the spinach in this experiment. (1 mark)

Solution

g. Rinsing is an essential step in the process for testing the cooked spinach. Explain why insufficient rinsing of the cooked sample before commencing Step 2 of Part A, above, would lead to a calculated C₂O₄^2- concentration that is higher than the true value. (2 marks)

Solution

h. Discuss whether the findings support Karolina’s hypothesis. (2 marks)

Solution