Some fruit contains large quantities of ascorbic acid (vitamin C). Ascorbic acid can be absorbed through the partially permeable wall of the gut. You will investigate the rate at which ascorbic acid from a 2gdm⁻³ ascorbic acid solution diffuses across a partially permeable membrane. Dialysis (Visking) tubing acts as a partially permeable membrane. You are provided with the materials shown in Table 1.1. Table 1.1 labelled | contents | hazard | quantity A | 2gdm⁻³ ascorbic acid solution | irritant | 80 cm³ W | distilled water | none | 200 cm³ D | dialysis tubing in distilled water | none | 20 cm I | iodine solution | irritant | 25 cm³ S | starch solution | none | 20 cm³ It is recommended that you wear suitable eye protection. You will need to: • allow ascorbic acid to diffuse out of the dialysis tubing into the distilled water surrounding the dialysis tubing • estimate the concentration of ascorbic acid that has diffused out of the dialysis tubing. Carry out step 1 to step 27. step 1 Tie a knot in the dialysis tubing as close as possible to one end, so that the end is sealed. step 2 To open the other end, wet the dialysis tubing and rub the tubing gently between your fingers and thumb. step 3 Put 10cm³ of ascorbic acid solution A into the open end of the dialysis tubing. step 4 Rinse the outside of the dialysis tubing by dipping it in the container of water labelled D. step 5 Carefully place the filled dialysis tubing bag into the large test-tube and secure in position using an elastic band, as shown in [Figure 1.1].

1. (a)(i)State the volume of distilled water you added. volume of water ................................................ cm³[1]
2. (a)(ii)Decide which other concentrations of ascorbic acid to make and complete Table 1.2. Table 1.2 concentration of ascorbic acid /gdm⁻³ | volume of ascorbic acid solution /cm³ | volume of distilled water /cm³ 0.0 | 0.0 | 20.0 2.0 | 20.0 | 0.0[2]
3. (a)(iii)Record your results in an appropriate table.[5]
4. (a)(iv)Record the volume of iodine added.[1]
5. (a)(v)Use your results in (a)(iii) and (a)(iv) to estimate the concentration of ascorbic acid in the solution outside the dialysis tubing bag. concentration of ascorbic acid ................................................[1]
6. (a)(vi)Calculate the rate of diffusion of ascorbic acid out of the dialysis tubing. Show your working. rate of diffusion ................................................[2]
7. (a)(vii)Identify two significant sources of error when finding the concentration of ascorbic acid in the sample from the large test-tube.[2]
8. (a)(viii)Suggest how you would make two improvements to this investigation.[2]
9. (b)(i)Plot a graph of the data in Table 1.3 on the grid in [Figure 1.3]. Use a sharp pencil.[4]
10. (b)(ii)Explain the change in water content of the melon cubes between 16 hours and 20 hours.[2]

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