To determine the empirical formula of an acid-oxalate by titration with standard potassium permanganate solution

Materials:

Potassium tetroxalate solution, 0.1M NaOH, 0.02M potassium permanganate, phenolphthalein.

Theory:

The empirical formula of the given acid-oxalate can be found by finding the mass of hydrogen and oxalate ions present in 1L of the sample solution. The latter two items can be found by the following reactions respectively:

H+(aq) + OH-(aq) –> H2O(l)

5C2O42-(aq) + 2MnO4-(aq) + 16H+(aq) –> 10CO2(aq) + 2Mn2+(aq) + 8H2O(l)

Mass of H+ in 1L of sample solution:

= 1st titration result x molarity of NaOH x 40 x RAM of H+

Mass of C2O42- in 1L of sample solution:

= 2nd titration result x molarity of KMnO4 x x 40 x RMM of C2O42-

Mass of K+ in 1L of sample solution:

= Mass of potassium tetroxalate – mass of C2O42- – mass of H+

Empirical formula of the acid oxalate:

Let the acid oxalate be KxHy(C2O4)

x : y : z = no. of moles of H+ : no. of moles of K+ : no. of moles of C2O42-

Procedures:

1. 25mL of the given potassium tetroxalate was pipetted into a conical flask.

2. Three drops of phenolphthalein was added as indicator and the mixture was titrated against 0.1M sodium hydroxide solution.

3. The titration was repeated once.

4. 25mL of the given potassium tetroxalate was pipetted into a conical flask.

5. 20mL of dilute sulphuric acid was added.

6. The contents of the flask was warmed to about 60�C and was titrated against 0.02M potassium permanganate solution, keeping the temperature of the solution at this level throughout the titration.

7. The titration was repeated once.

8. (1)The mass of hydrogen present in 1L of sample solution, (2)mass of oxalate ion present in 1L of sample solution and (3)the empirical formula of the acid oxalate was determined.

Observation:

Titration I:

Trial

1

2

2nd burette reading

18.8

37.65

1st burette reading

0

18.8

Vol. Of NaOH used

18.8

18.85

Average volume of NaOH used = 18.825mL

Titration II:

Trial

1

2

2nd burette reading

26.5

29.5

1st burette reading

0.5

4.1

Vol. Of KMnO4 used

26.0

25.4

Average volume of KMnO4 used = 25.7mL

Calculations:

Mass of H+ in 1L of sample solution:

= x 0.1 x 40 x 1.008

= 0.0759g

Mass of C2O42- in 1L of sample solution:

= x 0.02 x x 40 x 88

= 4.5232g

Mass of K+ in 1L of sample solution:

= 5.45 – 4.5232 – 0.0759

= 0.8509g

Empirical formula of the acid oxalate:

Let the acid oxalate be KH3(C2O4)2

x : y : z = no. of moles of H+ : no. of moles of K+ : no. of moles of C2O42-

= 0.8509/39 : 0.0759/1.008 : 4.5232/39

~ 1 : 3 : 2

So the Empirical formula of the acid oxalate is KH3(C2O4)2

Discussion:

Through this experiment, the empirical formula of the acid oxalate was found to be KH3(C2O4)2. However, according to the model answer provided, the actual empirical formula of the acid oxalate should be KH3(C2O4)3. So there must exist some sources of error, causing such inaccuracy.

Most probably, the two sources of error are on overheating of the mixture of potassium tetroxalate and sulphuric acid before the second titration, and the presence of impurities in the oxalate.

If the mixture of the potassium tetroxalate was overheated or even boiled, the oxalate will be oxidized by the oxygen in air, forming carbon dioxide. Therefore with less oxalate in the mixture, less permanganate will be consumed. The mass of the oxalate ion present in 1L of sample solution will be calculated to be less than the actual value. Error is thus created.

Also there may also be some impurities existing in the oxalate. For example, oxalate acids and potassium hydroxide, can all cause inaccurate experimental results.