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You are required to test known samples for the presence of ascorbic acid (vitamin C).
You are provided with
| labelled | contents | hazard | volume/cm3 |
|---|---|---|---|
| A | 0.1% ascorbic acid | irritant | 100 |
| W | distilled water | none | 100 |
| iodine | iodine in potassium iodide | irritant | 40 |
| S | starch | none | 20 |
| X | unknown sample | irritant | 20 |
| Y | unknown sample | irritant | 20 |
To find the concentration of ascorbic acid in A and W you are required to find the volume of iodine added to the test samples until the end-point is reached.
The drops of iodine will be added one at a time using a small syringe.
To practise releasing drops from a small syringe:
1. Fill the syringe with 2cm3 distilled water.
2. Hold the syringe over an empty test-tube as shown in Fig. 1.1 and push the plunger gently to release one drop.
3. Repeat this until you can release one drop at a time.
To find the concentration of ascorbic acid in A and W you will need to add drops of iodine until a blue colour appears which lasts for more than 10 seconds. This is the end-point.
You will be required to record the volume of iodine added.
Proceed as follows:
4. Put 1 cm3 of S into a test-tube.
5. Add 5 cm3 of the test sample (e.g. A) into the same test-tube.
6. Shake the test-tube gently to mix the contents.
7. Fill a small syringe with 2.0 cm3 of iodine.
8. Wipe off any iodine from the outside of the syringe with a paper towel.
9. Add one drop of iodine to the mixture of S and A as shown in Fig. 1.1.
10. Mix gently and if there is no colour change add another drop.
11. When the blue colour first appears, wait 10 seconds to see if the end-point has been reached. If the blue colour disappears then add another drop.
12. Repeat steps 9 to 11 until the mixture stays blue for at least 10 seconds.
13. Record the volume of iodine added.
14. Repeat steps 4 to 13 with sample W.
volume of iodine added for sample
A ...............
W ...............
Proceed as follows to find the concentration of ascorbic acid in samples X and Y.
You will need to dilute the 0.10% ascorbic acid to provide a range of known concentrations.
You will need to make up 20 cm3 of each concentration of ascorbic acid.
Table 1.1 shows how to make up two of the concentrations you should use.
Table 1.1
| volume of ascorbic acid/cm3 | volume of distilled water/cm3 | concentration of ascorbic acid/% |
|---|---|---|
| 20 | 0 | 0.10 |
| 16 | 4 | 0.08 |
(i) Decide which other concentrations of ascorbic acid to make and complete Table 1.2, including the concentrations from Table 1.1.
Table 1.2
| tube number | volume of ascorbic acid/cm3 | volume of distilled water/cm3 | concentration of ascorbic acid/% |
|---|---|---|---|
| [...] | [...] | [...] | [...] |
[3]
Make up the concentrations of ascorbic acid you have chosen.
Repeat steps 4 to 13 with each of your concentrations of ascorbic acid and the two samples X and Y.
(ii) Prepare the space below to show the concentration of ascorbic acid and record your results, including samples X and Y.
[6]
(iii) Plot a graph of the results.
[4]
(iv) Use your graph to estimate the ascorbic acid concentration of sample X.
Show clearly on your graph how you obtained the ascorbic acid concentration.
concentration of ascorbic acid in sample X ...............
[3]
(v) Identify two significant sources of error when finding the concentration of ascorbic acid in sample X.
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[2]
(vi) Suggest how you would make three improvements to this investigation.
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[3]
592
581
502
L1 is a slide of a stained transverse section through a leaf-like structure. You are not expected to have studied this material.
Fig. 2.1
(a) (i) Draw a large plan diagram of the sector shown in Fig. 2.1 to include the outline of two vascular bundles. No details of the internal tissues of the vascular bundles are required.
Label, with a C, the tissue containing chloroplasts.
[5]
(ii) Locate a large vascular bundle on L1. Using high-power, draw a large plan diagram to show one large vascular bundle in detail.
Label the phloem.
[5]
Fig. 2.2 is a photomicrograph of a transverse section of a leaf from a different plant species.
Fig. 2.2
(b) Calculate the ratio of the thickness of the layer labelled B compared to the total thickness of the layer labelled A as shown in Fig. 2.2.
You may lose marks if you do not show your working or if you do not use appropriate units.
[3]
(c) Prepare the space below so that it is suitable for you to record the observable differences between the specimens on slide L1 and in Fig. 2.2.
Record your observations in the space you have prepared.
[3]
(d) Describe how the observable features of the leaf section shown in Fig. 2.2 support the conclusion that this is a leaf from a plant growing in a dry habitat.
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[3]
542
500
500
