Determining Molecular Formulas of Organic Compounds, Lecture notes of Technology

Download Determining Molecular Formulas of Organic Compounds and more Lecture notes Technology in PDF only on Docsity! 6.9 Molecular Formulas How clean is the air in your home? Despite the best air-filtering technology available, indoor air pollution is unavoidable. Some outdoor pollutants follow you in as you enter your home. However, most indoor pollutants come from one of two sources. They are either produced by chemical processes in the home, such as home heating, or released— “off-gassed”—by products that we bring into the home. A newly installed synthetic carpet, for example, releases a cocktail of volatile organic compounds (VOCs) into the air (Figure 1). You will learn more about indoor air pollutants in Section 11.5. You might recall that organic compounds consist mostly of carbon and hydrogen. VOCs are compounds that readily become gases at room temperature. VOCs can be natural or synthetic. For example, ethyl ethanoate is a VOC used in some nail polish removers and glues. It has relatively low toxicity and also occurs naturally in wine. Other VOCs, such as methylbenzene (toluene), are more toxic. Methylbenzene is a solvent used in glues and some flooring products. Toxic VOCs need to be handled with care to minimize exposure. You may know someone who has allergies to “something in the air.” The detec- tion of indoor air pollutants is important in identifying possible causes of allergies. Fortunately, recent advances in analytical chemistry have made the detection of trace amounts of compounds in air possible. Scientists often use mass spectroscopy to detect and identify compounds in indoor air. As you learned in Section 6.8, a mass spectrometer is an analytical device that identifies compounds based on their molec- ular mass and characteristic fragment pattern. For example, Figure 2 shows the mass spectrum of a contaminant in indoor air. This readout indicates that the compound has a molecular mass of 88 u. 6.9 H C H H C O C C H H H O H H Re la tiv e in te ns ity Molecular mass (u) Mass Spectrum of Ethyl Ethanoate 0 40 60 20 10 30 70 50 10 20 88 � MCH3CO2CH2CH3 30 90 90 100 80 40 50 60 70 80 Figure 2 The largest molecular mass, 88 u, is the molecular mass of ethyl ethanoate. The other mass data are the masses of fragments of the molecule produced during the analysis. The molecular mass and fragment pattern together identify the compound. Figure 1 When new synthetic carpeting is installed, allow the area to air out for quite some time. This permits VOCs from the carpet and adhesives to escape. The Importance of Molecular Formulas Chemists use mass spectroscopy data together with an empirical formula to deter- mine the molecular formula of a compound. Remember that the empirical formula gives only the simplest ratio of the atoms or ions of each element in the compound. The molecular formula, however, gives the exact number of atoms of each element present (Section 6.7). When we use the term “chemical formula” for a molecular com- pound, we are referring to the molecular formula. Table 1 shows the molar mass and molecular formula of three organic compounds with the empirical formula C2H4O. However, only ethyl ethanoate has the molar mass 88.10 g/mol. 296 Chapter 6 • Quantities in Chemical Formulas NEL Table 1 Data for Identifying Three Organic Compounds with the Empirical Formula C2H4O Compound Molar mass (g/mol) Molecular formula Structural formula ethanal (acetaldehyde) ~ used in the production of particle board 44.05 C2H4O H C H H C O H ethyl ethanoate (ethyl acetate) ~ a solvent in glue 88.10 C4H8O2 H C C O C C H H H O H H H H 2, 4, 6-trimethyl-1,3,5- trioxane (paraldehyde) ~ a central nervous system depressant, once used as an anesthetic 132.15 C6H12O3 O CH CH CH O OH3C CH3 CH3 Note that the molar masses of ethyl ethanoate and paraldehyde are whole-number multiples of the molecular mass of the empirical formula C2H4O. Th at explains why the subscripts in their molecular formulas are also multiples of the subscripts in C2H4O. For example, since 3(44.05 u) 5 132.15 u, the molecular formula of paralde- hyde must be C233H433O133, which equals C6H12O3. In this activity you will construct molecules with the same empirical formula but different molecular formulas. Equipment and Materials: molecular model kit 1. Construct 3 molecules of methanal (formaldehyde), CH2O. Close the model kit box. For the rest of the activity, you will use only the atoms in these 3 molecules. 2. Take apart the methanal molecules and use some of the atoms to construct a molecule of a compound that has the molecular formula C2H4O2. Note the number of starting atoms that are left over. Record the structural formula for this compound. 3. Using exactly the same atoms, construct a different molecule with the same molecular formula: C2H4O2. Record the structural formula for this compound. 4. Construct a molecule with the molecular formula C3H6O3. Record the structural formula for this compound. 5. Rearrange the atoms from Step 4 to form a different molecule. Record the structural formula for this compound. A. What is the molecular mass of one molecule of methanal? T/I B. How is the molar mass of methanal similar to its molecular mass and yet quite different? k/u C. Predict the molecular mass of each molecule that you made in Steps 2, 3, 4, and 5. Comment on your predictions. T/I D. Explain how the molecular masses of C2H4O2 and C3H6O3 can be predicted from the molecular mass of methanal. T/I E. Why is determining the molecular formula of a compound an inadequate method of identifying the compound? Suggest a better way of identifying the compound. T/I Comparing Molecules and Molecular Formulas Mini Investigation Skills: Predicting, Performing, Observing, Analyzing, Communicating SKILLS HANDBOOK A2.4 NEL 6.9 Molecular Formulas 297