Method
The solution of sodium thiosulphate was made by dissolving 6.12g of Na2S2O3.5H2O with 250 cm3 of distilled water. The sodium thiosulphate has a concentration of 0.1 mol.dm3. Set up the apparatus. Wash equipment with solution three times. The burette with the sodium thiosulphate, and the conical flask and the pipette with the iodine solution. Measure 25 cm3 of the standard solution into a conical flask, using a pipette. Add the starch as the indicator. Pour the sodium thiosulphate into a 50 cm3 burette. Pipette 25 cm3 portion of the iodine solution into a conical flaks. record the level. Drip the solution slowly into the conical flask, swirling the flask continuously. Rinse the wall of the reactor before the end point. Stop adding acid when a single drop turns the solution deep red to yellow. One drop accuracy. Record the new level of sodium thiosulphate in the burette. Calculate the volume of acid used. Carry out four titrations, one trial and three accurate. Give your answer to 2 decimal places.
Results
Calculations
Average volume of potassium manganate v11 used, from three accurate titrations:
29.65 + 29.60 + 29.70 = 29.65 cm3
Weighing board = 0.97
Altogether = 7.09
Na2S2O3.5H20
I2 + 2e- → 2I reduction
S2O32- + I2 → S4O62- + 2e- oxidation
2 S2O32- + I2 → 2I- + S4O62
Mass 6.12
Moles * mr 0.024 * 248
Moles 0.024
Molarity * vol/dm3 ? * 250/1000
= 0.096
=0.01
29.65 0.1 mol.dm3 S2O3 = 25.00 cm3 I2
2 S2O32- + I2 → 2I- + S4O62
1 mole S2O32- = 2I- + S4O62-
S2O32- =
0.1 * 29.65/1000 = 2.9 * 10-3 moles
2.9 * 10-3 moles = 1.48 * 10-3 moles
conc = 1.48 * 10-3 moles
25.00/1000 = 0.0593 mol.dm-3
Discussion
The end point might be different due to human error, they might have added a couple of extra drops or they might not have added enough drops. Or they might have lost some of the solution. Another factor instrument error which can be built in, due to it will not give a precise point. The pipette could have been a different scale e.g. 10, 15, 25 cm3 etc. which might have had some sodium hydroxide left in its tip. The reading could not be exactly clear due human usage of the instrument time after time. Précis accuracy. The numbers could have been blurred so you can not read it to. The meniscus of the solutions could be out a little. We could have lost some of the solution when we were transferring it from the pipette to the conical flask. It could have remained on the side of the pipette and the conical flask. We could not collect all the solutions no matter how hard we tried. Due to equipment error.
Instrument accuracy/ lowest reading * 100 = ans
0.05/ 28.40 * 100 = 0.17%
For the burette.
This shows that the equipment are not absolute accurate. Another factor could have been the air around us. Or standing near the window and the outside temperature could have its effect.
Other instruments could be used which have a better accuracy point. More sophisticated measuring equipment could also be another point to notice.
Conclusion
The morality of the iodine solution was 0.0593 moldm3. The results were close together and the indicator changed quickly at the end point. If you were to repeat this experiment you could make it better by using a different indicator instead of starch, or using a different concentration of sodium thiosulphate.
