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.
-------------------------------------------------------------------------------------------------------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.)
-------------------------------------------------------------------------------------------------------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. Add 4-6 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
VA04_acidOxalate/p.1 of 3
limited to 4 runs.
Procedure B: 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
Final burette reading/cm3
Initial burette reading/cm3
Volume of NaOH used/cm3
Average titrant:
(B) Titration with potassium permanganate
Concentration of potassium permanganate = __________
Colour change at end point = ________________
Run
Trial
1
2
Final burette reading/cm3
Initial burette reading/cm3
Volume of KMnO4 used/cm3
Average titrant:
3
3
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.
VA04_acidOxalate/p.2 of 3
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.
VA04_acidOxalate/p.3 of 3