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Chapter 4

Chemical Composition

Shaun Williams, PhD

The Mole and Molar Mass

The Mole

Molar Mass

\[ \text{Mass of 1 mol of S} = 1\,mol \times 32.07\,\bfrac{g}{mol} = 32.07\,g \] \[ \text{Mass of 2 mol of O} = 2\,mol \times 16.00\,\bfrac{g}{mol} = 32.00\,g \] \[ \text{Mass of 1 mol of } \chem{SO_2} = 64.07\,g \]

Percent Composition

Percent Composition by Mass

  • An expression of the portion of the total mass contributed by each element
  • To find the percent composition of E (E is any element):
The percentage breakdown of a generic compound.
\[ \text{%E} = \frac{\text{mass of E}}{\text{mass of sample}} \times 100% \]

Conversion with Molar Mass and Avogadro's Number

\[ 1\,mol = 6.022 \times 10^{23}\, atoms \]

Determining Empirical and Molecular Formulas

Empirical and Molecular Formulas

Example of Empirical and Molecular Formulas

Two molecules both having the same empirical formula, CH, but with very different molecular formulas, C6H6 and C2H2.

Empirical and Molecular Formulas - Practice

For which of these substances is the empirical formula the same as the molecular formula?

A series of molecules with different formulas.

Some Empirical and Molecular Formulas

Substance Molecular Formulas Empirical Formulas
cyclopentane \( \chem{C_5H_{10}} \) \( \chem{CH_2} \)
cyclohexane \( \chem{C_6H_{12}} \) \( \chem{CH_2} \)
ethylene \( \chem{C_2H_4} \) \( \chem{CH_2} \)
hydrogen sulfide \( \chem{H_2S} \) \( \chem{H_2S} \)
calcium chloride This compound does not have a molecular formula \( \chem{CaCl_2} \)

Finding Empirical Formulas

Finding Empirical Formulas - Continued

  1. Repeat for all elements in the compound.
  2. Find whole number subscripts by:
    1. Dividing the moles of the each element by the smallest number of moles. The quotients will give whole numbers which are now the subscripts for the empirical formula.
    2. If #1 does not give whole numbers, then multiply all numbers by a multiplier that will resolve the quotients into whole numbers.

Determining Molecular Formulas

  • To determine a molecular formula, the problem must give a piece of experimental data, such as a molar mass, MM.
  • To find the molecular formula:
    1. Find the empirical formula first.
    2. Divide the empirical formula’s molar mass by the experimental molar mass (which is given).
Comparison between molecular and empirical formulas and their molar masses.

Determining Percent Compositions Using Molar Mass

\[ \%E= \frac{MM(E) \times \text{# of moles E in compounds}}{\text{Total MM of compound}} \times 100\% \]

Chemical Composition of Solutions


Making a Solution

A picture sequence of pouring a solid into a flask, adding water, and stirring to make a solution.

Solution Concentration

Effect of Solution Concentration on Color

A picture of less and less concentrated blue solutions. As the concentration drops, the solution becomes less and less blue.

Determining Concentration

Dilution and the Dilution Equation

A picture of diluting a solution. Some concentrated solution is extracted, put into a new flask, water is added to get the neccessary concentration.

Molecular View of Dilution

A molecular view of diluting a solution. Some concentrated solution is extracted, put into a new flask, water is added to get the neccessary concentration.