Determination of an Empirical Formula and % Composition

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1 Chem 110 Lab Clark College Determination of an Empirical Formula and % Composition Percent composition will be discussed in your text, lecture and in lab. This concept is often used to determine how many grams of an element might be produced when a compound is decomposed, or how many grams of an element is necessary to produce a given quantity of compound (in grams). By using the mole relationship to get mass/number conversion factors, it is possible to determine the number of moles of water present per mole of copper sulfate. LEARNING OBJECTIVES After completing this experiment, you should feel comfortable with: The identity, location and use of more laboratory equipment. Experimentally determining the mole amounts of species in a compound. Using the electronic balances Synthesizing data to determine the formula of a compound. A. Definition Elements combine in definite ratios to form compounds. For instance, one atom of carbon combines with two atoms of oxygen to form carbon dioxide. In order to properly carry out a chemical reaction, a researcher must know how much of one starting material (i.e., an element or compound) will react with how much of another material (i.e., another element or compound) to produce some compound. If the starting materials are not present in the correct ratios, some starting material would not react and then would be left over (excess). Today you will perform a reaction and investigate the combining ratios of certain starting materials. From the information obtained in the experiment you will then determine the formula of the compound. B. Calculating Percent Composition Percent literally means parts per 100. Since we rarely work with exactly 100 items, we usually must calculate what the value would be. This is accomplished quite easily by use of the equation shown below. Part % = x 100 Total To calculate the percent composition (by mass) of each element in a compound: 1. Determine the compound s molar mass (or formula mass) by adding up the atomic masses of each element and multiplying that mass by the number of atoms of that element. Use the periodic table for the atomic mass. 2. Divide the total mass of each element by the compound s total mass. 3. Multiply that fractional mass by 100. Percent Composition Sp10 AEM Page 1 of 8

2 Example I: Find the mass percent of oxygen in one mole of H 3PO 4. Solution: Mass H : 3 mole H Mass P :1mole P Mass O : 4 mole O Total Molar mass of 1.01g = 3.03g H 1mol H 30.97g = 30.97g P 1mol P 16.00g = 64.00g O 1mol O compound = 98.00g Mass O %O = 100% Total mass 64.00g = 100% 98.00g = 65.31% O Using the values above, calculate the % by mass of H and also the % by mass of P in H 3PO 4: The percent composition values can be used to calculate the number of grams of an element in a given mass of compound. Example II: Calculate the number of grams of oxygen that would be present in a 4.00 gram sample of phosphoric acid, H 3PO 4. Solution: From the previous example, 65.31% of phosphoric acid (H 3PO 4) is oxygen by mass. This means that there are grams of O in grams of the compound phosphoric acid. Therefore: 65.31g O 4.00g H PO g H 3PO 4 3 = 2.61g O Percent Composition Sp10 AEM Page 2 of 8

3 C. Experimental Considerations In this experiment, you will be heating a sample of solid magnesium in the presence of oxygen to create a new compound. Masses are taken before heating to determine the mass of the original sample (the metal) and after heating to determine the mass of the compound (the oxide). The difference between these two masses is equal to the mass of the oxygen (atoms) that have become bonded to the metal ion. Heating time and temperature are critically important for this experiment. If not enough heat is applied, some pure metal will remain. This experiment takes a lot of heat to adequately saturate the metal with oxygen. By-products are also formed which means that we must take care when heating to ensure that all of the metal turns into the metal-oxide compound. Using the measurement of weight changes in a chemical reaction, we will determine the correct formula for our metal oxide compound. In this experiment you will study the the formation of magnesium oxide (Reaction A). In this reaction you will be heating ( ) the reactants using your Bunsen burner to produce the desired products. Reaction A: x Mg (s) + y O 2(g) Mg xo y (s) In order to determine the formula for metal oxide, the same data requested in the procedure (steps 1 8) must be obtained. Keep in mind that step 6 equals the weight of the magnesium and step 8 represents the weight of the oxygen (as atoms, not as oxygen molecules) gained. Notice that the product has oxygen ions present in the formula (this is an ionic compound because there is a metal present). Since the mass of an atom or ion is based on the number of protons and neutrons (and NOT the number of electrons, the mass of a neutral oxygen atom will be the same as the mass of an oxygen ion. In this experiment, we will stop partway and add water to the crucible. The reason for adding water in the magnesium oxide experiment and then heating can be explained as follows. Magnesium reacts with oxygen in the air to form magnesium oxide. However, air not only contains oxygen (21%) but also nitrogen (78%), and magnesium will also react with nitrogen as well: 3 Mg + N 2 Mg 3N 2 Since we do not want magnesium nitride, we must convert it to magnesium oxide; water and heating allows us to do this. The magnesium nitride is converted to magnesium hydroxide with addition of water. Mg 3N H 2O 3 Mg(OH) NH 3 And finally, upon heating the hydroxide is converted into the desired magnesium oxide. Mg(OH) 2 Magnesium oxide + H 2O (g) Percent Composition Sp10 AEM Page 3 of 8

4 D. Experimental Procedure After you have washed the crucible and lid, use only tongs (not your hands) to handle them 1. Place a clean, empty crucible with lid in a clay triangle on a ring stand (as shown in diagram). Tilt the lid so that it is slightly ajar, then heat strongly (bottom of crucible should turn red) for about 3 minutes. Turn off the Bunsen burner, and use tongs to close the lid so that water from the air does not get inside the dry crucible. Allow the crucible and lid to cool (this should take 5 minutes). You can test to see if your crucible is cool enough by making a tent using your hands around the crucible (do not touch the crucible! Simply determine if the crucible is still radiating heat. If the crucible is still giving off heat, it is too hot to weigh! wait a few more minutes and then test using the tent again). (accessed 4/25/2010) 2. Using tongs, transfer the crucible and lid (still closed) to a wire gauze and carry them to the balance. It s okay if you need to remove the lid momentarily to transfer the crucible & lid separately to the wire gauze. Mass the crucible and lid (together) carefully to the nearest 0.01g and record the mass in the Data Table provided. 3. Obtain approximately 0.5 g Mg pieces or turnings. Place the magnesium into the crucible, put on the cover and weigh. Record the mass of Magnesium, crucible and lid in the Data Table. 4. Mount the crucible in the clay triangle and gently heat it for 1-2 minutes. In order to start the Mg oxidizing, it will then become necessary to heat the crucible more strongly (glowing red). Consult the instructor if in doubt. Heat the Mg strongly, with the lid on, for ten minutes. After 10 minutes, continue heating the Mg for an additional 10 minutes, opening the lid to let in air every 30 sec. 1 min. After this second 10-minute heating period, remove the lid off of the crucible and continue to strongly heat the Mg with no lid for 3 4 minutes. (note, this is a total of ~ 26 minutes of heating time: 2 min, then 10 strongly lid on, then 10 strongly lifting lid, then 3-4 min strongly with no lid) 5. The reaction is complete when no flashes of light occur in the Mg when the lid is off. If it flashes, heat some more. Once the reaction appears complete, replace the lid and heat the crucible and lid to a glowing red for 1-2 additional minutes, then allow the crucible and lid to cool (keep the lid on). Percent Composition Sp10 AEM Page 4 of 8

5 6. Once the crucible and lid have cooled to room temperature, add several drops of water to the oxide with a dropper. Try to wet the entire sample, but do not flood it. Note the odor. Very gently, heat the crucible with the lid on until the oxide appears dry, then heat strongly to ensure that the excess water is removed completely. This is the end of the first heating. 7. Allow the crucible and lid to cool and weigh it. 8. Repeat the heating one more time (steps 4 and 5). You can cut the heating times in steps 4 and 5 in half. Cool the sample and re-weigh (note, you do not need to repeat step 6 again). If your second heating is not within ± 0.02 g ask your instructor for assistance. 9. When you are done with the experiment, discard your Mg xo in the trash can, and clean your crucible. *Calculate your value for x and y. Before you leave the lab, you must give these values to the lab instructor, who will record the sign your lab. Percent Composition Sp10 AEM Page 5 of 8

6 Data Table Formation of the Metal Oxide 1. Mass of crucible + lid grams 2. Mass of empty crucible + lid + solid metal grams Observation of metal appearance: 3. Mass of crucible + lid + sample after 1 st heating (through step 6 in procedure) grams Observation of metal appearance after heating: 4. Mass of crucible + lid + sample after 2 nd heating. grams 5. final mass (re-write 2 nd heating mass or if a third heating needed to be done, record final mass from heating here). grams 6. Calculated mass of initial metal in sample 7. Final mass of compound after all heating complete. 8. Calculated mass of O atoms gained. 9. Experimental value of x. (2 sf) 10. Experimental value of y. (2 sf) 11. Experimentally determined formula: (2 sf in subscripts) 12. Actual formula (round subscripts from line 11 to nearest whole numbers) Initials: X: Y: Percent Composition Sp10 AEM Page 6 of 8

7 Observations: What odor did you smell upon the addition of H 2O? 1. Calculation for line 6: Determining the mass of the metal in the crucible: 2. Calculation for line 7: Determining the mass of the compound in the crucible: 3. Calculation for line 8: Determination of amount of oxygen taken in by metal: 4. Calculation for line 9: Determination of x and y in formula: Mg xo y. NOTE: you can do this by calculating % by mass first or since you have grams, you can directly proceed into the calculation for determining the empirical formula. Percent Composition Sp10 AEM Page 7 of 8

8 Follow up Questions 1. Why shouldn t you touch your crucible with your hands? 2. Given that magnesium oxide is an ionic compound, using the periodic table, what are the charges on Magnesium ion = Mg + Oxygen ion = O - Combining those ions to make a neutral compound, what is the correct formula Does the correct formula agree with the formula that you determined experimentally in lab? 3. Determine the mass percent of each element present in Ca(HCO 3) 2. Percent Composition Sp10 AEM Page 8 of 8

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