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Dissolve this in deionized water and make up to 250 cm3 in a volumetric flask.
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Use such iodate(V) solution to standardize the given thiosulphate solution as follows :
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Pipette 25.0 cm3 of KIO3(aq) into a conical flask, add to it about 5 cm3 of 1 M KI(aq) followed
by about 10 cm3 of 0.5 M H2SO4(aq) , then immediately titrate with 0.05 M Na2S2O3(aq) .
- Add a few drops of freshly prepared starch solution when the reaction mixture turns pale
yellow and continue to titrate to the end point. Record your results in Table 1.
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Dissolve the vitamin C tablet in about 150 cm3 of 0.5 M H2SO4(aq).
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Transfer the resulting solution to a volumetric flask & make up to 250 cm3 using deionized water.
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Pipette 25.0 cm3 of the vitamin C solution into a conical flask & add to it 5 cm3 of 1 M KI(aq).
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Pipette 25.0 cm3 of the standard KIO3(aq) into the flask containing vitamin C and KI(aq).
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Titrate the iodine content immediately with the standardized Na2S2O3(aq).
Record your results in Table 2.
Results : Mass of the Vitamin C Tablet used = _________4.58______________ g
Mass of weighing bottle & potassium iodate(V) = _____________5.5_____________ g
Mass of weighing bottle = _____________4.87____________ g
Mass of potassium iodate(V) used = _____________0.63____________ g
Molar mass of potassium iodate(V) = 214
Mole of potassium iodate(V) used = _______0.00294______ mole
Manufacturers’ specification = ______1000______ mg Vitamin C per tablet
Molar mass of Vitamin C = 176
Mole of Vitamin C content of the tablet = ___________5.68____________ mole
Mass of Vitamin C content of the tablet = __________0.659____________ g
Table 1
Mean volume of titrant (ml) = ________17.60_________
Table 2
Mean volume of titrant (ml) = _________4.33_________
Questions :
- What is the function of the starch solution? Why should it be added only when the reaction mixture becomes pale yellow?
When the reaction mixture becomes pale yellow, it is more difficult to recognize whether the reaction mixture to reach to the end point, the starch solution can let the reaction mixture color more apparent.
- Why is it preferable to carry out the titration immediately after the addition of sulphuric acid in step 3(b) ?
Because the iodine will be formed by redox reaction after sulphuric acid is added. But the iodine is volatile, so the concentration will be change to place it a time, and it will be affect the experimental results.
- It is well known that vitamin C deteriorates on heating & on exposure to air. Describe, giving experimental details, how you would investigate these 2 factors.
To test the vitamin C deteriorates on heating. We can do the following experiment. First, dissolve the vitamin C tablet in H2SO4, then pipette 25cm^3 of the solution in 2 conical flasks, one of them called solution A was heated, another called solution B was put in room condition. Add KI KIO3 to this 2 solution, then the titration was carried out. Calculation and record the mass of vitamin C. Comparing the mass of solution A and B, we can find that the mass of solution B is larger than solution A. Form this, we know that the vitamin C is deteriorated after heating with a high temperature under heating.
To test the factor of exposure to air. We can do the following experiment. First, dissolve the vitamin C tablet in H2SO4, then pipette 25cm^3 of the solution in 2 conical flasks, one of them called solution A was titrate immediately, another called solution B was titrate after a period a time. Calculation and record the mass of vitamin C. Comparing the mass of solution A and B, we can find that the mass of solution A is larger than solution B. Since solution B was titrate after a period of time, vitamin C mass dropped. It is due to the deterioration of vitamin C which is exposure in the air for the period of time.
- What is the mass of the ascorbic acid per tablet? How does the result compare with the manufacturers’ specification ?
The mass of the ascorbic is 659mg. Compare with the manufactures’ specification, it is only a little bit lighter. Hence I think the is manufactures’ specification quite accurate.
- Why is it preferable a tablet without flavouring & colouring ?
Because the flavouring may affect the actual mass of the main constituent. And the colouring may affect the color change in the titrate experiments.
- What is the term given to this kind of titration ?
It is called back titration. The reagents haven’t a suitable indicator for detecting the end point, so we need against the other reagents which it can be indicator for detecting the end point of titration.
- Vitamin C is an anti-oxidant. State another vitamin that is also an anti-oxidant.
Vitamin E. It can act as a natural antioxidant by scavenging free radicals and molecular oxygen. The vitamin E and C are interrelated in their antioxidant capabilities.