6_2 Empirical and Molecular Formulas.pdf - SECTION 6.2 Key...

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SECTION6.2Empirical and Molecular FormulasKey Termsmolecular formulaempirical formularatioIn Section 6.1, you read that chemists can determine the percentage composition ofa compound from mass data alone. However, the percentage composition only tellschemists which elements the compound contains and the relative masses of theseelements. For more specific information about the composition of a compound,you need to know the relative number of atoms of each element.Comparing Molecular and Empirical Formulas of CompoundsChemical formulas communicate which elements are found in a compound, as well astheir proportions. For example, the formula for hydrogen peroxide,H2O2, indicatesthat a molecule of hydrogen peroxide contains two atoms of hydrogen and two atomsof oxygen. Thus, this formula shows the actual number of atoms of each element inone molecule of the compound. Such a formula is known as themolecular formula.Molecular formulas can often be quite complex, as some molecules, such as glucose,C6H12O6, contain many atoms.A simpler method of writing a chemical formula involves showing only theproportional relationship or relative number of the atoms of each element in acompound. Such a formula, known as anempirical formula, shows the smallestwhole-numberratio, or proportional relationship, of the elements in a compound.For instance, the molecular formula for hydrogen peroxide isH2O2, and the ratio ofhydrogen to oxygen atoms is 2:2. The ratio 2:2 is not the smallest ratio because you canreduce it to 1:1. Thus, the empirical formula for this compound is HO,Note that, for some compounds, the empirical formula may be the same as themolecular formula. For example, water has two hydrogen atoms and one oxygen atom,as shown inFigure 6.6. Thus, the molecular formula for water isH2O. As this formulashows the smallest whole-number ratio of the elements in this molecule, it is also themolecule’s empirical formula. Additionally, it is important to recognize, as you learnedin Chapter 2, that ionic compounds only have one possible atomic configuration. As aresult, they are always represented by empirical formulas. They never have a molecularformula. As the name indicates, molecular formulas applyonly to molecules.Figure 6.6The empirical and molecular formulas for water are the same,as the molecularformula for water already shows the smallest whole-number molar ratio of the elements inthis molecule.Chemical Formulas and Specific CompositionThe empirical formula for a compound gives a very basic glimpse into the compositionof the compound. For this reason, the empirical formula is a very good start in anexperimental analysis to determine the formula for a new or unknown compound.

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Term
Winter
Professor
SINGH/PERRY
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Molecular formula

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