THE SYNTHESIS OF AN IRON SALT
EXPERIMENT A
INTRODUCTION
In this experiment, a complex iron salt with the empirical formula KxFe(C2O4)y·zH2O will be made and purified. The reaction is shown below. This will be done in groups assigned to you by your teaching assistant.
FeCl3 (aq) + K2C2O4 (aq) + H2O(l) ® KxFe(C2O4)y·zH2O + junk
Or
xK+(aq) + Fe3+(aq) + yC2O42-(aq) + zH2O ® KxFe(C2O4)y·zH2O(s) A-1
The ultimate goal is to find the empirical formula by determining x, y, and z, the moles of each component K, C2O4, and H2O respectively (mole ratio). These will be calculated from the percent by mass determined for of K, Fe, C2O4, and H2O in the experiments listed below:
1. The team’s procedure for the analysis of H2O. Analysis done during experiment D (3 trials).
2. Lab E – Analysis of K & Fe: Extension of the Acid-Base Chemistry Experiment (One or more trials should be done by each person in the group).
3. Lab F – Analysis for oxalate ion, C2O4: Extension of the Redox Chemistry Experiment (One or more trials should be done by each person in the group for a total of at least 3 trials).
All data should be immediately exchanged between group members. The calculations and report (interpretation) must be done individually. This approach is very similar to the way questions are answered in industry and research where different individuals do various aspects of projects and the results are combined to generate a solution.
EXPERIMENTAL
EQUIPMENT AND MATERIALS:
Two 100 mL beakers, spatula, wash bottle, 25 mL graduated cylinder, watch glass, a small wide-mouth amber bottle, filter paper, vacuum filtration setup (buchner funnel, rubber adaptor, filter flask, pressure tubing, clamp, ring stand), oven, and a dessicator.
CHEMICALS: The hazard code for each chemical is listed inside parentheses. The order is (health/toxicity, flammability, reactivity, and contact/corrosiveness).
- Potassium Oxalate, K2C2O4 · H2O (3013) - 24.0 grams per group
- Iron (III) chloride (Ferric chloride), FeCl3 (1012) - 16 mL of a 0.400 g/mL sol. per group
- Acetone, CH3COCH3 (1321)
- RO water and ice.
SAFETY CONCERNS:
Risk Assessment: Low to moderate due to acetone (flammable) and potassium oxalate (health hazard). All waste solutions except acetone may be safely discarded down the drain. Acetone should be collected in the bottle labeled “Waste Acetone” in the hood. ACETONE IS A FLAMMABLE SOLVENT; work with the acetone in the hood only. If a spill occurs outside the hood, make sure all burners and hot plates in the area are turned off. Sponge or mop up the spilled acetone or acetone mixture, and use large quantities of water to rinse it down the drain. Potassium oxalate is toxic, so avoid contact.
EXPERIMENTAL PROCEDURE: The Synthesis and Purification of an Iron Salt
First Lab Period [BPB1]– Synthesis, Crystallization, and Recrystallization of the Iron Salt
Synthesis:
n Weigh 24.0 g of K2C2O4·H2O (MW 184.2 g/mol) into a clean dry 100 mL beaker. Record all decimal places.
NOTE: Always record measurements to the correct precision and use glassware that will provide the needed precision while still providing the quickest measurement. The precision or amount of digits past the decimal can tell you what type of glassware or instrumentation to use. The table on precision of glassware in the introduction provides the precision of various types of glassware. In deciding what glassware to use, use the required precision, capacity of glassware, and time for measurement to make your decision.
For example, 10 mL could be measured with a 250 mL graduated cylinder because it has a precision of ±1 mL that is less than 10 mL volume to be measured but a 10 mL graduated cylinder would be much better with a precision of ±0.05 mL. They would both take about the same amount of time to use but the 10 mL would be much more precise. Any glassware that is more precise (it has a precision less than ±1 mL) could also be used. The only glassware that can’t be used to measure a 10 mL volume is a beaker. Its precision is ±10 mL, so measuring a 10 mL volume would not be useful and so the actual volume could range from ~5 to 15 mL. Side Note: a measurement of about 10 mL is a really sloppy measurement. A small beaker could be used for this.
Usually you want to obtain the best required precision in the least amount of time. For example, when measuring 10.0 mL of a liquid, a 10.00 mL volumetric pipet takes a lot longer to deliver the same liquid when compared to 10.0 mL from a 10 mL graduated cylinder. The pipet is also harder to clean, so the graduated cylinder would be the best choice to use.
You will also have the option of using a 5 or 10 mL pipettor for 1-10 mL transfers.
n Add about 60 mL of RO water, heat gently (hot plate setting of low or 2, each hot plate is different), and stir to dissolve. To stir, use a clean Teflon stir bar, and set the hot plate at a moderate speed. (Figure A-1.)
n While this is dissolving, clean a wash bottle, 25 mL graduated cylinder, 100 mL beaker, watch glass, and a wide mouth amber bottle. Your TA will provide the amber bottle. Fill the wash bottle with RO water.
n When the K2C2O4·H2O is dissolved add 16.0 mL of 0.400 g/mL FeCl3 and stir for about one minute and remove the stir bar. Use the magnetic stir bar retriever on the outside of the beaker to remove the stir bar (see TA for demo).
Crystallization:
n Remove the solution from heat and let cool for about 20 minutes. Exchange names and workstation/bin numbers (record in your notebook) while waiting.
n Cool the solution for another 40-60 min. in a slurry of ice and water. The slurry should be mostly ice. Avoid getting the ice slurry in your solution. The crystals will be an emerald green color.
Devise a procedure for determining the percent H2O while waiting. The final copy must be written in your lab notebooks and must be verified by your TA with his or her signature.
Helpful Hints: The procedure to determine the percent of H2O is very much like experiment 4, the hydrate experiment, in CHM 151L. Some things will need to be changed.
1. Notice that the procedure to determine percent H2O is not part of the synthesis and purification of the iron salt crystals. It determines one component (H2O) of the iron salt. So…Do not combine the crystals dehydrated in the percent water procedure with the rest of your crystals!
2. In the formula KxFe(C2O4)y·zH2O, the dot (·) symbolizes that H2O is the water of hydration. This means that the water is included in the lattice structure of the iron salt. Hydrogen bonds and ion-dipole attractions hold it in the crystal lattice. For these crystals, the water can be removed from the crystal lattice of the iron salt by heating to 120°C to form the anhydrous iron salt. Heat the crystals for about an hour. (WARNING: The crystals will start to decompose after about two hours.)
3. How much sample should you use? (Definitely do not use all of the crystals, remember that you have 3 trials for this analysis. There are two more experiments where the iron salt will be used. Together they use another 2 grams of iron salt.) 1.0-1.2g per trial is a good amount if you have enough sample. If the crystals are large you may need to grind the up a bit to ensure complete dehydration. Remember to weigh the beaker before weighing your iron salt in it.
4. Spread the crystals out in the bottom of a small beaker to ensure dehydration. Nest this inside a larger beaker to prevent contamination (by falling over, spillage, etc.) of the sample while in the oven.
5. The larger or outer beakers must be labeled with your name and section letter. Label the smaller beaker with a pencil on the white part of the beaker.
6. After heating, the dehydrated samples should be cooled for 5 minutes and then placed in a desiccator for about 15 minutes (or until they get to room temperature). The crystals must be cool to ensure that the balance reads the correct mass.
A desiccator is a seal-able container with a drying agent like CaSO4 or dierite in the bottom. The drying agent is an anhydrous compound that absorbs water, keeping the desiccator’s atmosphere dry.
7. Weigh the dehydrated crystals and repeat steps 2 and 6 to ensure that all the water is removed from the sample (Second heating should be for about 30 min.)
Purification:
n Decantation: After the crystals are formed, remove the liquid or solvent by pouring it into another container. If you have a good yield of crystals (at least 6 g or more) dispose of the filtrate by pouring it down the sink.
Figure A-2. Picture of the decanting process.
What is Decantation? This is a process by which the solvent is poured off of the crystals as shown in figure A-2. Removing the solvent at this point removes excess reactants and unwanted side products (junk) found in the solvent.
n Recrystallization:
What is recrystallization? A process by which a minimum amount of hot pure solvent used to dissolve the crystals and the solution is allowed to cool slowly such that the crystals formed are much more pure. The solution is cooled slowly to room temperature in the lockers to prevent impurities from being trapped in the crystal structure. This process removes impurities and results in the formation of larger crystals.
o Add about 20 mL of RO water to the crystals, break them up with a spatula, and gently heat and stir until they dissolve. Be gentle with your spatula to avoid cracking the beaker.
NOTE: If a dark residue remains undissolved, allow it to settle to the bottom, carefully decant the transparent green solution into another beaker, and discard the residue. If decanting doesn’t work try vacuum filtration to remove the residue.
If the crystals don’t dissolve try adding water (in 10 mL increments). Do not add more than an extra 30 mL. If this doesn’t work, see your TA
o Cover the beaker with a watch glass or parafilm and set in a bin until next lab. Each team member should record the bin# the crystals are in.
o Each group should clean a small, amber bottle, dry it, and store it in a group member's bin
n Before leaving, have TA sign and date notebook.
Second Lab Period – Vacuum Filtration, Drying, and Storage
NOTE: This will be done while doing Lab B (Calorimetry)
Purification (cont.):
n Vacuum Filtration.
NOTE: Vacuum filtration is used to separate a solid from a liquid. In this case we will separate the iron salt crystals from the aqueous solvent. Setup vacuum filtration apparatus shown below in figure A-3 if not yet setup. The pressure tubing is attached to the vacuum and filter paper placed in funnel. The vacuum filtration apparatus should be cleaned and left on the lab bench after it is used.
Figure A-3. Vacuum Filtration Apparatus.
o Cool a wash bottle of RO water in an ice bath.
o Make sure the vacuum filtration apparatus, a glass stirring rod, two 100 mL beakers, and one 250 mL beaker are clean.
o Put filter paper in the funnel, wet it with RO water (to create a good seal), and turn the vacuum on. You should be able to see the water being sucked out of the filter paper.
o Using the stirring rod or spatula, break up and pour the crystals onto the filter paper in the funnel.
o Rinse the beaker with about 25 mL of the cold water from the wash bottle. Mix this around with the stir rod and quickly pour over the crystals in the buchner filter funnel to rinse off impurities and transfer any remaining crystals. Do this one to two more times.
o Leave the vacuum filter on for 2 to 3 minutes until water is no longer dripping out of the funnel.
o Turn off the vacuum and pour the solution (filtrate) into a 250 mL beaker (do not mix this aqueous filtrate with acetone later).
Helpful Hint (2nd crop crystals): The filtrate should be saved in case you don’t get at least 6 grams of crystals. See your TA about this.
Drying Crystals:
n Take your buckner funnel with the crystals to the fume hood and place it in the filter flask setup found there.
n Transfer about 15 mL of acetone into a 50mL beaker using the pump dispenser in the fume hood. While the vacuum is on in the hood, wash the crystals with about 15 mL of acetone.
n Rinse the crystals again with 15 mL of acetone and leave for 1 to 2 minutes with the vacuum on to remove residual water and acetone from the crystals. Dispose of the acetone solution in the filter flask by placing it in the waste acetone bottle in the fume hood. (The acetone has a low boiling point and therefore vaporizes more quickly than water. More acetone than water on the crystals means the more quickly they will get dry.)
n Take the buchner funnel with the crystals back to your work bench and weigh the dry crystals in a 50 or 100 mL beaker. You should have at least 6 grams. If not see your TA about a 2nd crop crystals mentioned above.
n Weigh the small amber bottle that was cleaned last period and store the dried crystals in it with the lid off to the air to dry them completely.
NOTE: The crystals are stored in an amber bottle because they decompose when exposed to light.
n Before leaving, have TA sign and date notebook.
Third Lab Period – Reweighing to get final mass of iron salt crystals
n Reweigh the total amount of crystals
n During a later experiment, three or more simultaneous trials will be done to determine the percent water in the iron salt crystals your group made. You will use your percent water procedure to determine the percent water in the iron salt during a later experiment.
n Dispose of any filtrate solutions (except acetone) by washing them down the drain. There is a waste bottle for acetone in the hood.
n Before leaving have TA sign and date notebook.
Post-Lab Error Analysis: (This should be done when experiment A is completed)
After completing the last part of experiment A, comment on any problems with the procedure and answer the following questions in your laboratory notebook. How might the crystals be contaminated and what was done to prevent this? There are no calculations, but the crystals will be analyzed in later experiments to determine the percent by mass of the four components so that the empirical formula can be determined. List the four components you will analyze for in the iron salt, and do the following example empirical formula question in your lab notebook:
What is the empirical formula for a compound when a sample of it was found to contain 27.31% Ca, 48.06% Cl, and 25.43% H2O by mass? Is there a problem with the data? (you will need to do a similar calculation after you finish experiment F) Show your work.
Each team member will individually do the calculations for a formal scientific report (due later in the semester) on the synthesis and analysis of the KxFe(C2O4)y·zH2O crystals. For the report, only the data can be collected as a group. More information on the report will be provided at the end of experiment F.