Apparatus
50 cm3 burette, 20 cm3 pipettes, dropping pipette, tile, 250 cm3 measuring beaker, and 100 cm3 iron 11, 400 cm3 potassium manganate, clamp stand filter funnel, 250 cm3 conical flask.
Method
The solution of iron 11 has a concentration of 0.1 mol/ dm3. Set up the apparatus. Wash equipment with solution three times. The burette with the potassium manganate v11, and the conical flask and the pipette with the iron 11 solution. Measure 25 cm3 of the standard solution into a conical flask, using a pipette. The potassium manganate v11 acts as its own indicator. Pour the potassium manganate v11 into a 50 cm3 burette. Record the level. Drip the acid 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 colourless. One drop accuracy. Record the new level of acid 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:
28.40 + 28.55 + 28.45 = 28.47 cm3
Moles of Fe2+ = Concentration * volume in dm3
= 0.1 * 0.025
= 0.0025 moles
From the equation
MnO4- + 8H+ + 5e- → Mn2+ + 4H2O
5 moles of Fe 11 = 1 moles of MnO4
Concentration = moles / volume
0.0125 moles of Fe 11 = 0.0025 moles of MnO4
Concentration of MnO4 = moles / volume
= 0.0025/ 28.47/1000
= 0.0025/ 0.02847
= 0.087 moldm3
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 potassium manganate v11 was 0.087 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 e.g. methyl orange, indicators are weak acids, where the acid and its conjugate bases are different colours. The ph of the indicators changes over 2 units. Or using a different concentration of iron II.
