The concept of chemical composition is introduced through the analogy of buying nails by weight instead of counting individual pieces.
Similar approach is applied when dealing with atoms, specifically carbon atoms in this context.
Calculating Number of Atoms Using Mass
Nail Example: If one nail weighs 5 grams and a total weight of nails is 280 grams, then:
Number of nails = ( \frac{280}{5} = 56 )
Carbon Atoms Example:
Atomic mass of carbon: 12.01 atomic mass units (amu)
Conversion factor from amu to grams: 6.022 x 10^23 amu = 1 gram
Mass of one carbon atom in grams: ( \frac{12.01}{6.022 \times 10^{23}} \approx 1.994 \times 10^{-23} ) grams
For 8.25 grams of carbon: ( \frac{8.25}{1.994 \times 10^{-23}} \approx 4.14 \times 10^{23} ) carbon atoms
The Concept of a Mole
Definition: 1 mole = 6.022 x 10^23 entities (Avogadro's number)
Similar to the concept of a dozen (12), but much larger
Avogadro's number is used to relate the macroscopic scale (grams) to the microscopic scale (atoms/molecules).
Periodic Table and Moles
Atomic mass from the periodic table represents:
Grams per mole of atoms
E.g., 12.01 grams/mole for carbon
Converting Between Units
Diagram:
Moles ↔ Grams
Moles ↔ Atoms (or Molecules)
Example Conversions:
Grams to moles (e.g., 9.75 grams of sulfur to moles)
Moles to grams (e.g., 41.28 moles of sulfur to grams)
Moles to atoms (using Avogadro's number)
Molecule and Compound Calculations
Water Example:
Molecular weight of water (H2O): 18.02 amu or grams/mole
Grams to Moles for Compounds:
Sodium chloride (NaCl) example
Determining grams of sodium in a given amount of NaCl
Mass Percent Composition
Table Salt (NaCl) Example:
Sodium makes up 39.34% of NaCl by mass
Water (H2O) Example:
Oxygen mass percent is 88.79%
Hydrogen mass percent is 11.21%
Empirical and Molecular Formulas
Example Problem with Titanium Oxide:
Empirical formula determination through mass ratio calculation
Use of molar mass to determine molecular formula
Empirical vs. Molecular Formula:
Empirical formula shows simplest ratio
Molecular formula provides actual number of atoms present
Conclusion
Understanding of chemical composition is crucial for converting between mass, moles, and atoms, and for determining empirical and molecular formulas of compounds.