Titration To Determine the Concentration of a Solution of Potassium Maganate

The volume of potassium manganate v11 needed to neutralize the iron 11. You simply titrate the potassium manganate v11against the solution of iron 11and acid with a known concentration. Then when you have calculated the concentration of the acid you can use it in turn as a standard solution, to find the concentration of the solution of alkali. (Acids are substances which donate protons and the bases are substances which accept them)

MnO4- + 8H+ + 5e- –> Mn2+ + 4H2O

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(Purple) (H2So4) (Colourless) (Water)

Reduction Mno4- , Mn+, Mn+7.

Oxidation Fe2+ –> Fe3+ + 1e-

The electron transfer must be the same…

5Fe2+ –> 5Fe3+ + 5e-

Safety

The sulphuric acid is corrosive. The potassium manganate is strongly oxidising. At all times through the experiment you must wear eye protection and lab coats. When filling the pipette be careful it does not break. And when filling the burette make sure it is below eye level. If any spillages have been made clean them up and report it.

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

Titrations

Initial reading

Final reading

volume used cm3

1

0.00

28.55

28.55

2

0.00

28.40

28.40

3

0.00

28.55

28.55

4

0.00

28.45

28.45

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. Pr�cis 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.

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