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

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Introduction

Experiment 12 Date: 26th January, 1999. Object: To determine the empirical formula of an acid-oxalate by titration with standard potassium permanganate solution. Apparatus: 25mL measuring cylinder, 250mL beaker, 250mL conical flask (x2), burette, 25mL pipette, pipette filler, stand and clamp, white tile, safety goggles. 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 ...read more.

Middle

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. ...read more.

Conclusion

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. ...read more.

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