Determining Molecular Formulas from Empirical Formulas and Molar Masses, Schemes and Mind Maps of Chemistry

Download Determining Molecular Formulas from Empirical Formulas and Molar Masses and more Schemes and Mind Maps Chemistry in PDF only on Docsity! Empirical formulas don't distinguish between molecules with different molecular formulas that happen to have the same percent composition. Check out these molecules, which all have the empirical formula of CH: C2H2 C3H3 C4H4 C5H5 C6H6 26.04 g/mol 39.06 g/mol 52.08 g/mol 65.10 g/mol 78.11 g/mol You can distinguish these molecules by their molecular mass, though. If you know the molecular mass of your compound (which you can determine with an instrument called a mass spectrometer), then you can figure out which of these compounds you have! 1. Determine how much one “unit” of your empirical formula weighs 2. How many of those formula units fit into the given molecular mass? 3. Multiply the subscripts in the empirical formula by the number you found in Step 2. Example The empirical formula of ribose (a sugar) is CH2O. In a separate experiment, using a mass spectrometer, the molar mass of ribose was determined to be 150 g/mol. What is the molecular formula of ribose? 1. The “molar mass” of the empirical formula CH2O is 12.01+2(1.01)+16.00=30.03 g /mol 2. The molar mass of ribose is 150 g/mol. There are 150 30 =5 “empirical units” in each ribose molecule. 3. Multiply each of the subscripts in the empirical formula (CH2O) by 5: C5H10O5 Therefore, the molecular formula of ribose is C5H10O5. Molecular Formula from Empirical Formula ChemistNATE Questions 1. The empirical formula of butane, the fuel used in disposable lighters, is C2H5. In an experiment, the molar mass of butane was determined to be 58 g/mol. What is the molecular formula of butane? 2. Oxalic acid has the empirical formula CHO2 . Its molar mass is 90 g/mol. What is the molecular formula of oxalic acid? 3. The empirical formula of codeine is C18H21NO3. If the molar mass of codeine is 299 g/mol, what is its molecular formula? 4. A compound’s molar mass is 240.28 g/mol. Its percentage composition is 75.0% carbon, 5.05% hydrogen, and 20.0% oxygen. What is the compound’s molecular formula?