F.6/7 Chemistry Practical: Empirical Formula of an Acid-Oxalate

Objective: To determine the empirical formula of potassium hydrogen oxalate

Group size: Individual

Introduction:

In this experiment the number of moles of hydrogen ions obtained from one mole of the acid-oxalate is determined by the titration against standardized sodium hydroxide solution, using phenolphthalein as the indicator. The number of moles of oxalate ion is determined by the titration of the acid oxalate against standardized potassium permanganate solution. Since the concentration of the acid oxalate solution is known, number of moles of potassium ion in the acid oxalate solution can be calculated and the mole ratio of these ions can be determined.

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Theory:Give principle/reason to explain how do you determine the empirical formula of the salt, potassium acid oxalate in this experiment. The following questions should be useful to you.

  1. Assume the empirical formula of the salt, potassium acid oxalate be KxHy(C2O4)y. Give equation for (a) reaction of the acid oxalate and sodium hydroxide (b) reaction of the acid oxalate and acidified potassium permanganate by using ‘half-equation’ method.
  2. The average titrant for procedure A is V1 and the average titrant for procedure B is V2. Show, how do you derive the empirical formula by using these data.

(N.B. Suppose molarity of sodium hydroxide is C1 while that of KMnO4 is C2.)

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Chemicals:

200 cm3 of KxHy(C2O4)z.2H2O [i.e. 7.85 g of the crystals dissolved in 1 dm3 solution], 120 cm3 0.1 M NaOH solution, 150 cm3 of 0.025 M KMnO4 solution, 100 cm3 of 1.0 - 1.2 M sulphuric(VI) acid,

Bench solutions: potassium hydrogen oxalate, potassium oxalate, phenolphthalein indicator

Additional Apparatus:

Thermometer, 0-110 oC, 50 cm3 burette, tripod, burner and ceramic plate

Safety precaution:

Sodium hydroxide and sulphuric acid are corrosive, potassium permanganate is oxidizing, potassium hydrogen oxalate is harmful

Procedure:

Pre-requisite experiment:

The action of the indicator is investigated by adding a drop of phenolphthalein solution, in turn, to a few cm3 of solutions of potassium hydrogen oxalate, potassium oxalate and sodium hydroxide. Give your observations in a tabular form.

Obtain 200 cm3 of aqueous solution of an acid-oxalate KxHy(C2O4)z.2H2O from your teacher. [How can you obtain the sample without changing its concentration?] Record the concentration of the solution.

Procedure A: To determine number of hydrogen in potassium hydrogen oxalate.

  1. Pipette 25.0 cm3 of the acid-oxalate solution into a 250 cm3 conical flask
  2. Add4-6 drops of phenolphthalein indicator to the acid solution and titrate the acid solution against 0.1 M sodium hydroxide solution.
  3. Record your readings in a tabular form. Try to get two concordant readings. However, you are limited to 4 runs.

<big>Procedure B</big>: To determine number of oxalate ion in potassium acid oxalate

  1. Pipette 25.0 cm3 of the acid-oxalate solution into a 250 cm3 conical flask and add about 25 cm3 of approximate 1 M sulphuric(VI) acid.
  2. Warm the contents of the flask to about 60 oC and titrate against potassium permanganate solution (appr. 0.25 M), maintaining the temperature of the solution at approximate 60 oC throughout the experiment.
  3. Repeat the titration until two concordant readings are obtained.

Data and Results:

Pre-requisite experiment:

potassium hydrogen oxalate / potassium oxalate / sodium hydroxide
Colour of phenolphthalein

Titration Data

(A)Titration with sodium hydroxide solution

Concentration of sodium hydroxide solution = ______

Run / Trial / 1 / 2 / 3
Final burette reading/cm3
Initial burette reading/cm3
Volume of NaOH used/cm3

Average titrant:

(B) Titration with potassium permanganate

Concentration of potassium permanganate = ______

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Run / Trial / 1 / 2 / 3
Final burette reading/cm3
Initial burette reading/cm3
Volume of KMnO4 used/cm3

Average titrant:

Guidelines for calculation:

  1. Calculate (a) number of moles of hydrogen ions in 25.0 cm3 solution and (b) grams of hydrogen in 1 dm3 of the acid-oxalate solution.
  2. Calculate (a) number of mole of oxalate ions in the 25.0 cm3 solution and (b) grams of oxalate in 1 dm3 acid-oxalate solution.
  3. Calculate grams of potassium present in 1 dm3 acid-oxalate solution.
  4. Taking the formula mass, H=1, C=12, O=16, K=39, calculate the empirical formula of the acid-oxalate.

Discussion

  1. Is methyl orange a suitable indicator in procedure A? Explain your answer.
  2. When the oxalate solution is titrated against acidified potassium permanganate(VII), the temperature of the reaction mixture is kept at about 60 oC. Explain.
  3. Potassium permanganate(VII) is employed here as a primary standard to titrate the reducing agent oxalate ion. Comment on the suitability of using this reagent.
  4. List safety precautions for this experiment.
  5. Give error sources and estimate percentage error for (a) mass of hydrogen, (b) mass of oxalate and (c) mass of potassium.

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