Abstract

As chemists create new compounds, they use a variety of instrumental and experimental techniques to determine the composition of the new substances. By determining what is in the compound and in what ratios, they can then determine the chemical formula. The term "empirical formula" means based on observation and experiment and is used to describe the ratio of elements in a compound.  In this lab, you will convert iron metal (an element) to iron oxide (a compound). Based on the weights of reactant and product, you can find the percent of iron and oxygen in the compound. This percent of iron and oxygen is known as the percent composition. Since metals combine with oxygen in different ratios, you can then find the empirical formula of the iron oxide compound you synthesized. The empirical formula will take the form of Fe_xO_y, and you are trying to find the numerical value of x and y for the iron oxide compound. 

Bring the necessary chemicals to your lab bench and perform all reactions in the bench hood. 

1. Wash and dry a ceramic evaporating dish. Check the dish for any cracks or chips. If you find any, go to the Prep Room and ask for a new dish.
2. To make sure that the evaporating dish is completely dry, you will need to heat it with the Bunsen burner. Set up your ring stand with a clay triangle (not wire gauze) placed on the ring. Make sure that the ring stand is reasonably clean and free of any flakes of iron or paint, as these particles may contaminate your reaction. Rest your evaporating dish securely on the triangle. Start heating the dish with a gentle flame and then gradually increase the flame intensity to maximum heat (a non-yellow flame). Continue to flame the dish for 5-10 minutes. Remove the flame and allow the dish to cool to room temperature. Once you are sure the dish is cool, mass it. 

Record the mass of the empty evaporating dish.

3. While you are heating and cooling the evaporating dish, obtain a piece of iron wire that weighs approximately 0.1g. Rub the wire with steel wool to remove the oxidized coating from the iron wire. Coil it into a flat coil about 2 inches in diameter. Record the mass of the coil to the nearest 0.001g. Place the coil of iron in a 400mL beaker (make sure that it lies flat on the bottom).

In the presence of water, iron combines with oxygen in the air to form rust. This reaction is one of the most common and costly examples of a metal being oxidized.

4. In the bench hood (bring the HCl to the lab bench), carefully add 10.0mL of concentrated HCl (hydrochloric acid) to the iron wire. Cover the top of the beaker with a watch glass and allow the iron to dissolve completely. When the initially vigorous reaction slows down, you may heat the mixture gently to dissolve the iron [NOT TO A BOIL!]. All the iron must be completely dissolved; check your solution carefully for any remaining particles before proceeding.

Heat the mixture gently IN THE BENCH HOOD!

5. Let the beaker cool and then rinse the surface of the watch glass into the beaker to wash any iron that may have boiled up onto the glass. Add distilled water to the solution until the beaker is half full. To the solution, add 5.0 mL of 3% H₂O₂ (hydrogen peroxide) and cover the beaker again with the watch glass. Place a stirring rod in the opening between the beaker and the watch glass, as this will prevent bumping and splattering when the solution is heated. Bring the mixture to a boil and continue to boil for 5 minutes. This heating promotes the decomposition of the hydrogen peroxide, which is evident by the bubbles of oxygen that form (be sure not to confuse the oxygen bubbles with the boiling). If oxygen is still being generated after five minutes, continue to heat until bubbles are no longer seen.
6. Cool the beaker to room temperature and rinse the watch glass into the beaker as before. Add approximately 2 g of ammonium nitrate to the solution and stir until the ammonium nitrate has dissolved. Prepare a solution of 15.0 mL concentrated ammonium hydroxide (NH4OH) in 30.0 mL distilled water. [Do not walk around with a beaker of conc. NH₄OH. Dilute the conc. NH₄OH with 30 mL of water.]
7. Again heat the iron solution to the boiling point and add the ammonia solution slowly, with stirring. Continue to add the ammonia until a precipitate forms and persists, and then add 1-2 mL additional ammonia solution. Describe the appearance of the precipitate. It should clump together and settle to the bottom. If it does not, add some more ammonia.
8. While allowing the precipitate to settle, prepare a funnel with filter paper for gravity filtration.

   For gravity filtration, you must first prepare the filter. Obtain a piece of the appropriate size filter paper. Fold the circle in half to form a semicircle, and crease the fold carefully (do not make too sharp a crease, as this will be likely to tear). Fold the semicircle in half again and you should now have a quarter circle. Tear off a small piece of one of the corners. This will improve the seal between the filter paper and the funnel. The figure below demonstrates the folding of filter paper.

   

   Fit the filter to the conical funnel by opening the quarter circle into a cone. Make sure that the cone is opened in a way such that three folds are on one side and one is on the other side. Also, make sure that the torn corner is one of the three folds (this will ensure a good fit; it is critical that the filter fits snugly so that no product leaks and the filtration proceeds rapidly). Holding the cone in the funnel, wet the paper with distilled water from your squirt bottle and gently press the filter paper to the funnel. Make sure that there are no air bubbles between the paper and the funnel. Place the funnel in a ring attached to a ring stand, and make sure that the stem of the funnel rests against the side of the beaker that you are using to collect the filtrate (the liquid from the mixture). Decant as much of the supernatant (liquid) as possible though the funnel, and then collect the solid on the funnel. Be sure not to contaminate the product with any debris such as dust or paper. Rinse the sides of the beaker with a small amount of distilled water and add this washing to the product in the funnel. Repeat the rinsing process until you have transferred as much product as possible to the funnel.

9. When the product has drained thoroughly, carefully remove the filter paper containing your product and place the paper and solid in your previously weighed evaporating dish.
10. Place the dish back on the clay triangle and heat to dry the product and burn away the paper. Be careful not to spatter and lose the product. If your filter paper ignites, remove the burner and cover the evaporating dish with a watch glass to extinguish the fire. Once all the paper has been burned away, increase the flame of the burner to full heat and ignite your product for 10-15 minutes until it glows red.
11. Allow the dish to cool completely and weigh the dish and product. In order to be sure that all water has been removed from the product, you must dry the dish and product to constant weight. To do this, heat the dish and product again gently for 5 minutes; cool and weigh again. If the two final weights differ by more than 0.005g, repeat the heating and cooling until your results are consistent. Using your final value for the mass of the product and dish, determine the mass of the iron oxide.

Post-Experiment Analysis.

Include the following information in a lab report to give to your instructor.

1. The following measurements should be recorded in a data table:

• initial mass of iron
• final mass of iron oxide

2. Assuming that all the iron was converted to iron oxide, and that the only elements in your product are iron and oxygen, determine the percent composition of your iron oxide product.  The amount of iron in your compound will then be the same as the amount of iron that you started with, or the initial mass of iron. The amount of oxygen present will be the difference between the final weight of iron oxide and the initial mass of iron. The percent composition of each element is then:

Percent Composition of Iron Oxide

3. Determine the empirical formula of iron oxide. To calculate the empirical formula from the percent composition, assume you have 100g of the iron oxide. See sample exercise 3.12 in BLB.

4. Is your compound iron (II) oxide or iron (III) oxide. Explain.

5. Compare the physical properties of the iron oxide to the initial iron metal. In what way does the product differ from the reactant?

5. Why is it important to reach a constant weight in determining the amount of product obtained?

6. List three possible sources of experimental error in this experiment. Describe how each of these errors would effect your results.