Experiment 7 Empirical Formulas forma s Ken Karp Crucibles are fired at high temperatures to volatilize impurities. . OBJECTIVES To determine the empirical formulas of two compounds by combination reactions To determine the mole ratio of the decomposition products of a compound The following techniques are used in the Experimental Procedure: TECHNIQUES 6 12 136 15a 15b 15€ INTRODUCTION Data Analysis, A The empirical formula of a compound is the simplest whole-number ratio of moles of elements in the compound. The experimental determination of the empirical formula of a compound from its elements requires three steps: 1. Determine the mass of each element in the sample. 2. Calculate the number of moles of each element in the sample. 3. Express the ratio of the moles of each element as small whole numbers. For example, an analysis of a sample of table salt shows that 2.75 g of sodium and 4.25 g of chlorine are present. The moles of each element are mol Na mol C1 2.75 g X * = 0.120 mol Na, 4.25 g X = 0.120 mol C1 22.99 g Na 35.45 g Cl The mole ratio of sodium to chlorine is 0.120 to 0.120. As the empirical formula must be expressed in a ratio of small whole numbers, the whole-number ratio is 1 to 1, and the empirical formula of sodium chloride is NaCl, or simply NaCl. The empirical formula also provides a mass ratio of the elements in the compound. The formula NaCl states that 22.99 g (1 mol) of sodium combines with 35.45 g (1 mol) of chlorine to form 58.44 g (1 mol) of sodium chloride. The mass percentages of sodium and chlorine in sodium chloride are: 22.99 35.45 x 100 = 39.34% Na x 100 = 60.66% CI 22.99 + 35.45 22.99 + 35.45 We can determine the empirical formula of a compound from either a combination reaction or a decomposition reaction. In the combination reaction, a known mass of one reactant and the mass of the product are measured. An example of a combination reac- tion is the reaction of titanium with chlorine: The initial mass of the titanium and the final mass of the titanium chloride oxide product are determined. From the difference between the masses, the mass of chlorine that reacts and subsequently, the moles of Combination reaction: two elements combine to form a compound Experiment 7 115 Ý 12:08 PM Studypool Inc. 1 of 3 Formal Lab Reports should be typewritten and turned in to the lab instructor at the next laboratory period. Formal lab reports should be comprised of the following parts (for a more detailed description see page ix in your laboratory manual): 1. Introduction/Purpose 2. Procedure 3. Data, Observations, & Calculations 4. Discussion and Answers to lab questions 5. Conclusion An example laboratory report format is shown on the following page. Each of the FORMAL labs will be graded on a 20-point scheme. Neatness does count. Another person should be able to follow both what was done and the results that were obtained simply by looking at your completed report. If your instructor judges the work to be messy, illegible, or if the work does not include all of the required components, he or she may give a grade of zero without the opportunity to resubmit the work. Points will be deducted for insufficient detail. This is especially important in the procedure section of you lab. You must provide a clear and detailed explanation of what YOU did to complete the lab assignment. Points will be deducted for copying verbatim the procedure as described in the lab manual. Although you may be assigned to work in teams of two or more during the lab session, each person must submit their own lab report for each lab. While it is important for lab teams to discuss the results and formulate answers to the lab questions, each person must write the results and answers in their own words. Copying will not be tolerated. Reports which are similar or the same in detail will have points deducted from all team members. This policy is to dissuade students from writing one lab report and submitting it with different names, or for one student to copy sections from another members report. Page 4 of 10 Example Formal Laboratory Report Format Name: Date: Lab Section: e.g. Tues 11:30 – 2:00 are measured. An ratory Assignment question 2). example Control air access or more compounds titanium to chlorine yields the empirical formula of the titanium chloride (see Prelabo- titanium and chlorine that react to form the product are calculated. This mole ratio of on a compound analysis and the final mass of at least one of the products In the decomposition reaction, the initial mass of the compound used for the would be the decomposition of a mercury oxide to mercury metal and oxygen gas: The initial mass of the mercury oxide and the final mass of the mercury metal are reacted to form the mercury oxide. The moles of mercury and oxygen in the original determined. The difference between the measured masses is the mass of oxygen that compound are then calculated to provide a whole-number mole ratio of mercury to In Part B of this experiment, a combination reaction of magnesium and oxygen is used to determine the empirical formula of magnesium oxide. The initial mass of the In Part C of this experiment, a decomposition reaction of a pure compound into calcium oxide, CaO, and carbon dioxide, CO,, is analyzed. The masses of the com- pound and the calcium oxide are measured. Further analysis of the data provides the In Part D of this experiment, a combination reaction of tin and oxygen is used to determine the empirical formula of a tin oxide. Since tin forms more than one oxide, a dif- ference in laboratory technique and persistence may lead to different determinations of the reported empirical formula. The initial measured mass of tin is not reacted directly with the oxygen of the air, but rather with nitric acid to produce the oxide. Because the mass of the final product consists only of tin and oxygen, its empirical formula is calculated. oxygen (see Prelaboratory Assignment question 1). Clay triangle magnesium and the mass of the product are measured. oose a different position mole ratio of Cao to CO, in the pure compound. Jo A. Beran Figure 7.1 Controlling access of air to the magnesium ribbon Procedure Overview: A crucible of constant mass is used to thermally form (Part B) able composition (Part D). Mass measurements before and after the heating procedure TAL or decompose (Part C) a compound of fixed composition or form a compound of vari are used to calculate the empirical formula of the compound. Ask your instructor which part(s) of the experiment you are to complete. If you are to perform more than one part, then you will need to organize some of your data on a separate sheet of paper for the Report Sheet. Be aware of the number of significant figures when recording data. If too much air comes in contact with the Mg ribbon, rapid oxidation of the Mg occurs and it burns brightly. (Caution: You do not want this to happen. If it does, do not watch the burn; it may cause temporary blindness!) Immediately return the lid to the crucible, allow the apparatus to cool, and return to Part A. I to repeat the experiment. 3. Heat for complete reaction. Continue gently heating the crucible until no visible change is apparent in the magnesium ash at the bottom of the crucible. Remove the lid; continue heating the open crucible and ash for -30 seconds. Remove the heat and allow the crucible to cool to room temperature. Do not touch! Measure the mass of the contents in the crucible and with the lid on the same balance that was used earlier and record. 4. Repeat. Redo Parts A.1 and B.1-3 for a second trial. 5. Calculations. Determine the mole ratio of magnesium to oxygen, and thus the empirical formula, of the pure compound. 8. Data Analysis, A 1. Prepare a clean crucible. Obtain a clean crucible and lid. Often-used crucibles tend to form stress fractures or fissures. Check the crucible for flaws; if any are found, obtain a second crucible. Support the crucible and lid on a clay triangle and heat with an intense flame for -- 5 minutes. Allow them to cool to room temperature. Measure the mass of the fired, cool crucible and lid. Use only clean, dry cru- cible tongs to handle the crucible and hid for the remainder of the experiment. Do not use your fingers. (Caution: Hot and cold crucibles look the same-do not touch!) 1.2 Disposal and cleanup: Wash the cool crucible with a dilute solution of hydrochloric acid and discard in the Waste Acids container. Rinse twice with tap water and twice with deionized water. action of ygen 1. Prepare the sample. Polish (with steel wool or sandpaper) 0.15-0.20 g of magne- sium ribbon, curl the ribbon to lie in the crucible. Measure and record the mass (+0.001 g) of the magnesium sample, crucible, and lid. 2. Heat the sample in air. Place the crucible containing the Mg ribbon and lid on the clay triangle. Heat slowly, occasionally lifting the lid to allow air to reach the Mg ribbon (Figure 7.1). 156, The pure compound decomposes to calcium oxide and carbon dioxide. Complete Part C. Decomposition Reaction A.1 for preparing the crucible for analysis. of a Pure Compound 150 1. Dry the compound. The pure compound is to be previously dried before analysis. Check with your instructor. 2. Prepare the sample. Place about 1 g of the pure compound in the clean crucible prepared in Part A.1 and measure the mass (+0.001 g) of the sample and crucible. 6 The lid need not be used for Part C. Record the mass of the compound. Cool the crucible and lid to room temperature (and perform all other cooling processes in the Expert imental Procedure) in a desiccator if one is available. When cool, remove the crucible and lid from the desiccator with crucible tongs and measure the mass of the crucible and lid. Af the crucible remains dirty after heating, on the advice of your instructor, move the apparatus to the fume hood, add 1-2 ml of 6 M HNO,, and gently evaporate to dryness. (Caution: Avoid skin con Place the crucible and lid in a desiccator (if available) for cooling. tact, flush immediately with water.) Experiment 7 117 ulas T 12:09 PM Studypool Inc. Example Formal Laboratory Report Format Date: Lab Section: e.g. Tues 11:30 – 2:00 Name: Title of Experiment:. I. Introduction/ Purpose: This should be one or two sentences that explain(s) what you plan to do during the laboratory experiment. Example This experiment will determine the accuracy of a 10 mL graduated cylinder at its 10 mL mark based on the known density of water at a given temperature. Also in this experiment, the density of an unknown solution will be determined by measuring the mass and the volume of the solution and calculating the density. II. Procedure:A summary of the experimental procedure used in the lab. This is to be in your own words, not copied from the laboratory manual. Do not include laboratory observations in the procedure section. This section is typically written in the past tense third person. Do not write outlines, numbered lists, or bulleted lists. This section must be written in paragraph form. III. Data, Observations & Calculations: This section will contain all the observations, raw data, and calculated values with clearly performed examples from the experiment. For a formal lab report, the data sheet found within the lab manual is NOT SUFFICIENT. You must provide an organized data table of your own design and details of all the observations and calculations involved. Chemical reactions involved in the experiment should be written out in this section along with all masses and volumes of reagents. Percent yields must be calculated where appropriate. IV. Discussion: This section should be written in paragraph form and must account for the observations and data you obtained from the experiment. You should summarize your results and relate them to the chemistry that you know. Each lab background section contains concepts to consider as you write this section. V. Conclusion This section of the lab report describes what you have learned in the experiment. Summarize your observations and results. If the laboratory involves the determination of an unknown, include the unknown number and the results that you obtained. Use this section of the lab report too discuss the purpose of the experiment, any sources of errors that you encountered, the reliability of the data, additional investigations that you might suggest, and the relationship between your discoveries and chemical principles, theories, and concepts. If your experiment included the determination of Page 5 of 10 + 12:09 PM Studypool Inc. 2 of 3 III. Data, Observations & Calculations: This section will contain all the observations, raw data, and calculated values with clearly performed examples from the experiment. For a formal lab report, the data sheet found within the lab manual is NOT SUFFICIENT. You must provide an organized data table of your own design and details of all the observations and calculations involved. Chemical reactions involved in the experiment should be written out in this section along with all masses and volumes of reagents. Percent yields must be calculated where appropriate. IV. Discussion: This section should be written in paragraph form and must account for the observations and data you obtained from the experiment. You should summarize your results and relate them to the chemistry that you know. Each lab background section contains concepts to consider as you write this section. V. Conclusion This section of the lab report describes what you have learned in the experiment. Summarize your observations and results. If the laboratory involves the determination of an unknown, include the unknown number and the results that you obtained. Use this section of the lab report too discuss the purpose of the experiment, any sources of errors that you encountered, the reliability of the data, additional investigations that you might suggest, and the relationship between your discoveries and chemical principles, theories, and concepts. If your experiment included the determination of Page 5 of 10 some value which can be compared to a known actual value, your must include a calculation of percent error and write a conclusion about the accuracy of your results. TOPICAL OUTLINE
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