10 Empirical Formulas POGIL

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10 Empirical Formulas POGIL.docx Page 1 of 8 Empirical Formulas POGIL How can the chemical formulas of a compound be determined? Transparency Guidelines Purpose: • We are completing this POGIL exercise to introduce Concept Area VI: Empirical Formulas Chapter 2. o We will propose the empirical formula for a compound when the molecular formula is provided. o We will determine the empirical formula of a compound from percent composition data. o We will discuss the limitations of an empirical formula in identifying a unique compound. • We will also be practicing many of the various POGIL skills while working in our teams; these are the skills that employers want in their future employees. Task: • The teams will complete the handout with each team member taking on the appropriate role(s). If an in- person cl ass at each stopping point, each team’s spokesperson will share the team’s answers for discussion with the class. On-line sections, the manager should use the suggested times to keep the team moving forward. All roles use the stop signs as a place to pause to check in with all teammates to ensure everyone is ready to proceed. • The teams will complete the handouts observing the models and reading the information given. This will allow students to complete the learning cycle: explore, invent, and apply. • For students that have never seen this material before, it is expected that there will be some initial frustration and confusion while completing the activity. However, through discussion with teammates, at the end students will have a solid understanding of empirical formulas. Criteria: • Teams will know they are on the correct path when they are able to reach consensus about what an answer should be within the available time. Teams may reach out to the instructor as she walks around (or for on- line classes, reach out to the instructor via your favorite method) if unable to reach a consensus so that she can help clear roadblocks. • A team is successful if they completed the questions within the allotted time, their answers match the answers shared by other teams during the report outs (or for online classes your team reaches consensus), and the team worked well together as defined by their POGIL roles, and the teamwork reflection is completed. And now, the POGIL: Why? Scientists use chemical formulas as a shorthand method of communicating with each other about the make-up and structure of compounds. There are several types of formulas that are used to convey different types of information. This activity will compare two types of useful formulas.

10 Empirical Formulas POGIL.docx Page 2 of 8 Model 1 – Empirical Formula Compound Name Molecular Formula Ratio of C Atoms to H Atoms Lowest Whole Number Ratio of C Atoms to H Atoms Empirical Formula methane CH 4 1 : 4 1 : 4 CH 4 ethene (ethylene) C 2 H 4 2 : 4 1 : 2 CH 2 propene C 3 H 6 CH 2 ethyne (acetylene) C 2 H 2 2 : 2 1 : 1 CH benzene C 6 H 6 CH 1,3-butadiene C 4 H 6 C 2 H 3 propane C 3 H 8 C 3 H 8 1. Consider the information in Model 1. How are the columns “ratio of carbon atoms to hydrogen atoms” and “lowest whole number ratio of carbon atoms to hydrogen atoms” related ? 2. Complete the table in Model 1 by filling in all the blank spaces. 3. Which set of ratios in Model 1 is most closely related to the empirical formulas of the compounds – the ratio of the carbon to hydrogen atoms or the lowest whole number ratio of carbon atoms to hydrogen atoms? 4. According to Model 1, is the empirical formula for a compound always different from the molecular formula of a compound? If no, describe a case when these two formulas would be the same. 5. Imagine you are helping a friend who missed class today. Briefly explain to your friend how to write the empirical formula for a compound when given the molecular formula. 6. Could a specific compound be identified uniquely by its empirical formula? Justify your answer with specific examples from Model 1. 7. Write the empirical formulas for each of the following compounds. C 5 H 10 C 4 H 10 N 2 O 5 P 4 O 10 SO 2 ~10 minutes

10 Empirical Formulas POGIL.docx Page 3 of 8 Model 2 – Percent Compositions Compound Name Molecular Formula Moles C in 1 Mole of Compound Moles H in 1 Mole of Compound Grams C in 1 Mole of Compound Grams H in 1 Mole of Compound C % Mass H % Mass methane CH 4 1.00 mol 4.00 mol 12.0 g 4.00 g 75.0% 25.0% ethene (ethylene) C 2 H 4 2.00 mol 2.00 mol 24.0 g 4.00 g 85.7% 14.3% propene C 3 H 6 3.00 mol 6.00 mol 36.0 g 6.00 g 85.7% 14.3% ethyne (acetylene) C 2 H 2 benzene C 6 H 6 1,3- butadiene C 4 H 6 propane C 3 H 8 8. Consider the data in Model 2. Discuss as a team how the values were obtained in each column, then divide the work for completing the remaining four compounds. Show your work below for ethyne, benzene, 1,3- butadiene, or propane. 9. Refer to Model 2. What do you notice about the percent composition of compounds with the same empirical formulas? 10. Could a specific compound be identified uniquely by its percent composition? Justify your answer with specific examples from Model 2. ~10 minutes

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10 Empirical Formulas POGIL.docx Page 4 of 8 11. Use the data in Model 2 to perform the following calculations by first discussing as a team how the calculations would be done, and then dividing the work among team members. List your answers in the table below. Several of the calculations have been done for you as examples. Molecular Formula Empirical Formula Lowest Ratio of Moles C : Moles H Lowest Ratio of Mass C : Mass H Lowest Ratio of % C : % H CH 4 CH 4 1 : 4 3 : 1 3 : 1 C 2 H 4 CH 2 1 : 2 6 : 1 6 : 1 C 3 H 6 CH 2 1 : 2 6 : 1 6 : 1 C 2 H 2 C 6 H 6 C 4 H 6 C 3 H 8 a. List the lowest whole number ratio of moles of carbon in 1 mole of compound to moles of hydrogen in 1 mole of compound. b. Calculate the lowest whole number ratio of mass of carbon in 1 mole of compound to mass of hydrogen atoms in 1 mole of compound. Show your work! c. Calculate the lowest whole number ratio of carbon percent mass to hydrogen percent mass for each compound. Hint : If the lowest whole number ratio is not clear by inspection, divide both values by the lesser of the two numbers. Show your work! d. Which of the ratios in the table above correlates best to the empirical formula of the compound? e. Do any of the ratios in the above table match? If so, which ones? Read This! When chemists are working with a new or unknown compound, one method they can use to determine the chemical formula of the substance is to determine the percent composition. There are several methods for chemically breaking down a compound and finding the relative masses of the elements in the substance. These data are then used to determine the empirical formula of the substance. Although this does not provide the molecular formula, it is a step in the right direction. ~15 minutes

10 Empirical Formulas POGIL.docx Page 5 of 8 12. Consider the four samples of methane below. According to Model 2, the percent composition for methane is 75.0% carbon and 25.0% hydrogen. Use this information to fill in the table below. Divide the work among team members. Show your work for each calculation for one column. Total Mass of Methane Sample 100.0 g 72.0 g 12.0 g 10.0 g Grams of C Atoms Moles of C Atoms Grams of H Atoms Moles of H Atoms Lowest Whole Number Ratio of Moles C : Moles H 13. Refer to your work in Question 13. a. Can the empirical formula of a compound be derived mathematically from percent composition data? Justify your answer. b. Would you need to know the mass of the sample when determining a compound’s empirical formula from percent composition data ? 14. List the steps needed to find the empirical formula for a compound when given the percent composition. 15. An unknown compound contains 50% sulfur and 50% oxygen. Determine the empirical formula of the compound. Show mathematical calculations to support your answer. ~10 minutes

10 Empirical Formulas POGIL.docx Page 6 of 8 16. Explain why the empirical formula of the compound in Question 15 is not SO even though its percent composition is 50 : 50. 17. An unknown compound contains 43.6% phosphorus and 56.4% oxygen. Determine the empirical formula of the compound. Show mathematical calculations to support your answer. 18. Do we know the molecular formula of either of the compounds in Questions 15 or 17? Explain your reasoning, Read This! The empirical formula of an unknown compound can be derived from percent composition data, but you still will not know which compound you have. You will, however, have narrowed down the possibilities. For example, if you know the empirical formula of a compound is C 2 H 3 , the molecular formula might be C 2 H 3 , C 4 H 6 , C 6 H 9 , C 8 H 12 , etc. In order to determine the molecular formula of a compound, you must know the compound’s molar mass. This will allow you to determine which multiple of the empirical formula is the correct molecular formula. 19. The molar mass of the compound in Question 17 is 283.88 g/mole. a. Use complete sentences to explain how this information could be used to determine the molecular formula of the compound. b. Now follow your steps and calculate the molecular formula. 20. Nicotine is a stimulant found in the tobacco plant. The percent composition is: 74.03% C, 8.70% H and 17.27% N. a. Determine the empirical formula of nicotine. ~10 minutes

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10 Empirical Formulas POGIL.docx Page 7 of 8 b. The molar mass of nicotine is 163.23 g/mole. Using this information and your answer to part a, calculate the molecular formula of nicotine. Model 3 – Isomers Name & Structural Formula Molecular Formula Empirical Formula Physical & Chemical Information 1-hexene CH 3 – CH 2 – CH 2 – CH 2 – CH=CH 2 3,3-dimethyl-1-butene cyclohexane 21. Determine the molecular formula and empirical formula for each of the compounds in Model 3; place your answers in the table. 22. In all cases, can a specific compound be identified uniquely by its molecular formula? Justify your answer with specific examples from Model 3. 23. The three substances in Model 3 are isomers. Write a definition for the term isomers. 24. What are the advantages and disadvantages of using structural formulas to represent a compound? CH 3 C CH 3 CH CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 ~15 minutes

10 Empirical Formulas POGIL.docx Page 8 of 8 25. Make a prediction. Do you think the isomers in Model 3 will have the same physical properties such as melting point, boiling point, and solubility? Explain your reasoning. 26. Make a prediction. Do you think the isomers in Model 3 will have the same chemical properties? Explain your reasoning 27. Find the three compounds in Model 3 in a textbook or on the Internet. a. List some of the physical and chemical properties that you find in the table; list your source(s) for the information below. b. Now, compare their physical and chemical properties. Are the three compounds the same substance or are they different substances? Justify your answer with information you find in your research.

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