GREEN CRYSTALLINE COMPOUND PART IV:
PERCENT OXALATE, EMPIRICAL FORMULA AND PERCENT YIELD
PURPOSE: This laboratory experiment is the fourth in a series of four experiments to
synthesize and then analyze a green crystalline compound. You will determine the
percent oxalate in the green crystalline compound with the general formula
KxFey(C2O4)z · a H2O, and then use this data and data from the previous three
experiments to obtain the empirical formula and calculate the percent yield of the
compound. In order to determine the percent oxalate, a redox titration will be conducted
using a standardized 0.0100 M potassium permanganate solution. The net ionic equation
for the reaction of oxalate ions (C2O42-) from the green crystalline compound with
permanganate ions (MnO4-) from potassium permanganate is:
16H+ + 5C2O42- + 2MnO4- ----------------> 10CO2 + 2Mn2+ + 8H2O
The appearance of a pink-purple color (that stays) locates the endpoint of the titration. A
massed sample of the green crystalline compound is used. Knowing the volume of MnO4solution required to reach the equivalence point, and its concentration, the moles of
permanganate can be determined.
moles MnO4- = V · M
V = volume in liters of KMnO4 solution needed to react with oxalate.
M = molarity of KMnO4 solution
The moles of oxalate and then the percent oxalate can be calculated from the moles of
permanganate.
moles C2O42- = moles MnO4- · 5 mol C2O422 mol MnO4grams C2O42- = moles C2O42- X 88.0 g C2O42mol C2O42% C2O42- = grams C2O42- X 100 %
---------------------------------------------grams of green crystalline compound
In order to calculate the empirical formula of the green crystalline compound, we usually
begin by assuming that we have 100 grams of the compound. That way the percentage of
each element converts directly to grams. The mass of each element is then divided by its
molar mass to find the number of moles of each element in the 100 gram sample. The
moles of the elements are in the same ratio as their subscripts in the empirical formula.
Once the empirical formula of the green crystalline compound is known, the theoretical
yield can be calculated. The mass of FeCl3 · 6H2O is divided by its molar mass. The
resulting number of moles of FeCl3 · 6H2O is multiplied by the mole ratio of moles of
KxFey(C2O4)z · a H2O to moles of FeCl3 · 6H2O. This quantity is multiplied by the molar
mass of the green crystalline compound to give the theoretical yield. The percent yield is
grams of the green crystalline compound collected divided by the theoretical yield with
the resulting quantity multiplied by 100.
PRELAB PROBLEMS:
1. A sample of the green crystalline compound with a mass of 0.150 gram (instead
of 0.125g) is transferred into an Erlenmeyer flask. 36.14 mL of 0.0100 M KMnO4
solution is required to titrate to the equivalence point. How many grams of C2O42are in the sample of the green crystalline compound and what is the percent
oxalate in the compound?
MATERIALS:
Buret and clamp
250 mL Erlenmeyer flask
100 mL graduated cylinder
10 mL graduated cylinder
.0100M KMnO4
Distilled water
6M H2SO4
green crystalline compound
85% H3PO4
hot plate
PROCEDURE:
1. Measure out about 0.125 gram (to the nearest 0.001 gram) of the green crystalline
compound into a 250 mL Erlenmeyer flask and record the precise mass on your
data sheet.
2. Add about 60 mL of distilled water to the flask to dissolve the crystals.
3. Add 6 mL of 6M H2SO4 to the green solution. The solution needs to be acidic.
4. Add 1 mL of 85% (concentrated) H3PO4 to the green solution. The Fe3+ ions give
the solution a yellow color, which will interfere with the endpoint if it is not
removed. The PO43- ions from H3PO4 react with Fe3+ forming colorless complex
ions.
5. Permanganate reacts very slowly with oxalate at room temperature. Heat the
green crystalline solution to near the boiling point on a hot plate. If the solution
starts to boil, take it off the hot plate.
6. Record the precise concentration of the 0.0100 M KMnO4 solution to three
significant figures on the data sheet.
7. Add three small rinse volumes (~3 mL) of the standardized KMnO4 solution to a
clean buret and drain each rinse solution into a waste container.
8. Fill the buret with the KMnO4 solution and run the solution out the tip to expel air
bubbles. Set the solution volume to the zero mark.
9. Titrate the solution of the green crystalline compound with the standardized
KMnO4 solution to the pink endpoint.
10. Record the volume of KMnO4 solution required to reach the endpoint.
11. Calculate the percent oxalate in the green crystalline compound.
12. Do a second trial and then calculate the average percent oxalate.
13. Determine the empirical formula of the green crystalline compound.
14. From your data from Part I of the Green Crystal Lab, determine the theoretical
yield and the percent yield for your green crystal.
DATA AND CALCULATIONS:
Include the following for both samples 1 and 2 in the data portion of your lab write
up:
A. Determination of percent oxalate:
Mass of green crystal compound, concentration of KMnO4 used, volume of KMnO4 used,
moles of KMnO4 used, moles of C2O4-2 used, mass of C2O4-2 used, % oxalate, average %
oxalate in both samples
B. Determination of the Empirical Formula of KxFe(C2O4)y · zH2O
Once you have calculated the % oxalate in the sample, using the % potassium from part
III of the lab, % iron from part III of the lab, and % water from part II of the lab, calculate
the empirical formula for your green crystal. These values You should round each
number to the nearest whole number; these numbers should be whole numbers from your
initial calculation, but will not be due to experimental error.
C. Determination of your theoretical and percent yield
Using your data from Green Crystal Part I Lab, calculate the theoretical yield and percent
yield of your total green crystal sample. Calculate the theoretical yield given the amounts
of FeCl3  6 H2O and K2C2O4 · 1H2O used in part I of the lab, and the following
equation:
1 FeCl3 ∙ 6 H2O + 3 K2C2O4 ∙ 1 H2O 1 K3 [Fe1(C2O4)3] ∙ 3 H2O
+ 3 KCl + 4 H2O
Your percent yield will be your actual yield/theoretical yield x 100%. Your actual yield
is the mass of the green crystal obtained in part I of the lab.
CONCLUSIONS:
1. Why does the solution go from colorless to pink/purple at the endpoint of the titration
of oxalate by the permanganate solution?
2. What is the limiting reagent in the reaction of FeCl3 · 6H2O with K2C2O4 · H2O to
form the green crystalline compound?