3.2 Determining Empirical and Molecular Formulas

3.2 Determining Empirical and Molecular Formulas

  • The bulk mass of a substance is related to the number of atoms in it.
    • We were able to determine the amount of the substance from its mass using the chemical formula.
  • In this section, we will look at how to apply the same principles in order to derive the chemical formulas of unknown substances.
  • Chemical formulas are the most succinct way of representing the makeup of a compound.
    • When a compound's formula is unknown, measuring the mass of each of its elements is the first step in the process of figuring it out.
    • Consider a compound composed of carbon and hydrogen.
  • The article was accessed on January 14, 2015.
  • The compound is 61.0% C, 15.4% H, and 23.7% N by mass, according to the analysis results.
  • A sample of a compound containing only carbon, oxygen, and chlorine has been determined to contain 3.01 g C, 4.00 g O, and 17.81 g Cl.
  • The relative abundance of a given element in different compounds of known formulas can be evaluated using percent composition.
    • Consider the common nitrogen-containedfertilizer ammonia (NH3), ammonium nitrate (NH4NO3), and urea.
    • The mass percentage of nitrogen in the compound is a practical and economic concern for consumers choosing among these fertilizers.
    • The percent composition of a compound can be derived from its formula mass and atomic mass.
    • A molecule of NH3 has one N atom weighing 14.01 amu and three H atoms weighing a total of 3 x 1.008 amu.
  • This same approach can be used to consider a pair of molecules, a dozen molecule, or a mole of molecule.
  • As long as we know the chemical formula of the substance in question, we can easily derive percent composition from the formula mass or molar mass.
  • There is a compound with the formula C9H8O4.
  • To calculate the percent composition, we need to know the mass of C, H, and O.
  • When rounded, these percentages equal 100.00%.
  • The most common approach to determining a compound's chemical formula is to first measure the mass of its elements.
    • The corresponding numbers of atoms in the compound must be derived from any data involving mass.
    • The mass of each element can be converted to a number of moles using the molar mass method.
    • The moles of each element relative to each other are converted into a whole-number ratio that can be used to derive the empirical formula of the substance.
    • A sample of compound contains 1.71 g C and 0.287 g H.
  • The formula for this compound is CH2.
  • A sample of compound is determined to contain 5.31 g Cl and 8.40 g O.
  • If we divide by the smallest subscript, we still have a decimal subscript in the empirical formula.
    • To convert this into a whole number, we must use the same atom ratio and formula as the final empirical formula.
  • To ensure that the smallest whole-number ratio of subscripts is obtained, the procedure is outlined in Figure 3.11.
  • The sample can be used to derive the empirical formula of a compound.
  • A sample of the black mineral hematite an oxide of iron found in many iron ores has 34.97 g of iron and 15.3 g of oxygen.
  • Hematite is used in jewelry.
  • Each element has a mass in grams.
  • The iron to oxygen ratio is 1.000 mol to 1.500 mol.
  • The formula is Fe2O3.
  • There are more examples of the derivation of empirical formulas that you can watch.
  • Consider instances in which a compound's percent composition is available rather than the absolute mass of the compound's elements.
    • In such cases, the percent composition can be used to calculate the mass of elements present in any convenient mass of compound; these mass can then be used to derive the empirical formula in the usual fashion.
  • The copper pipes at the top of the fermentation tanks are used to remove the oxide of carbon from the tanks.
  • Since the scale for percentages is 100, it is most convenient to calculate the mass of elements present in a sample weighing 100 g.
  • The empirical formula is CO2 since the resulting ratio is one carbon to two oxygen atoms.
  • Various measurement techniques can be used to determine these quantities.
    • The mass spectrum of the compound can be used to derive the molecule mass.
    • Many of the methods used to measure molar mass will be introduced in later chapters of this text.
  • An empirical formula mass is the average atomic mass of all the atoms in an empirical formula.
  • Consider a compound with an empirical formula that is determined to be CH2O.
    • The formula mass for this compound is 30 amu, which is the sum of 12 amu for one C atom, 2 amu for two H atoms, and 16 amu for one O atom.
  • The molecule's subscripts are six times greater than those in the empirical formula, which is why this approach is used.
  • We are only considering one mole of empirical formula units and molecule, as opposed to single units and molecule.