Lecture 4
Chemical Composition
Shaun Williams, PhD
The Mole and Molar Mass
The Mole
- The unit that acts as a bridge between the microscopic world and the macroscopic world
- Contains \( 6.022 \times 10^{23} \) particles (molecules, atoms, ions, formula units, etc.)
- This number is called Avogadro's number.
- The amount of substance that contains as many basic particles (atoms, molecules, or formula units) as there are atoms in exactly 12 g of carbon-12
Molar Mass
- Describes the mass of 1 mole of a substance
- We obtain the Molar Mass (MM) from the periodic table by assigning different units to the atomic mass.
- Instead of assigning the atomic mass units of amu, we assign the atomic mass units of grams per 1 mole.
- Molar mass is the conversion factor between mass and moles.
\[ \text{Mass of 1 mol of S} = 1\,mol \times 32.07\,\bfrac{g}{mol} = 32.07\,g \]
\[ \text{Mass of 2 mol of O} = 2\,mol \times 16.00\,\bfrac{g}{mol} = 32.00\,g \]
\[ \text{Mass of 1 mol of } \chem{SO_2} = 64.07\,g \]
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):
\[ \text{%E} = \frac{\text{mass of E}}{\text{mass of sample}} \times 100% \]
Conversion with Molar Mass and Avogadro's Number
- To convert from moles to grams or from grams to moles, use a molar mass (MM) as your conversion factor.
- To convert from moles to particles (molecules, atoms, ions, or formula units) or from particles to moles, use Avogadro's number as your conversion factor.
\[ 1\,mol = 6.022 \times 10^{23}\, atoms \]
Determining Empirical and Molecular Formulas
Empirical and Molecular Formulas
- Empirical formula
- Expresses the simplest ratios of atoms in a compound
- Written with the smallest whole-number subscripts
- Molecular formula
- Expresses the actual number of atoms in a compound
- Can have the same subscripts as the empirical formula or some multiple of them
Example of Empirical and Molecular Formulas
Empirical and Molecular Formulas - Practice
For which of these substances is the empirical formula the same as the molecular formula?
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 |
\( \chem{CaCl_2} \) |
Chemical Composition of Solutions
Solutions
- are any homogeneous mixture at the molecular or ionic scale
- are composed of solutes and solvents
- Solutes
- Are present in a lesser amount
- The substances that are dissolved (can be either wet or dry)
- Solvents
- Are present in the larger amount
- The substances that dissolve
Making a Solution
Solution Concentration
- Is the relative amounts of solute and solvent in a solution
- When compared with one another, solutions are classified as dilute or concentrated.
- Dilute solution
- A solution that contains a relatively small amount of solute
- Concentrated solution
- A solution that contains a relatively large amount of solute
Effect of Solution Concentration on Color
Determining Concentration
- Percent by Mass
- Expresses concentration via percentage
\[ \%\, mass = \frac{\text{mass solute}}{\text{mass solution}} \times 100\% \]
- Molarity (M)
- The moles of solute dissolved in 1 L of solution
- The most common units of concentration
\[ M = \frac{\text{moles solute}}{\text{Liters of solution}} \]
Dilution and the Dilution Equation
- Dilution
- The process of adding more solvent to solution
- The dilution equation:
\[ M_{con} V_{con} = M_{dil}V_{dil} \]
\[ M_1V_1 = M_2V_2 \]
Molecular View of Dilution
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