Overview
This lecture covers chemical calculations involving atoms, moles, and mass, emphasizing practical problem-solving and the concept of parts-to-whole in compounds.
Sodium Intake and Chemical Compounds
- Sodium in our diet is consumed as ionic compounds like NaCl, not as pure sodium metal.
- Table salt (NaCl) consists of both sodium and chloride ions, not just sodium.
- The FDA recommends limiting sodium intake to under 2300-2400 mg/day.
Ratios and Parts-to-Whole Calculations
- Chemical calculations often involve determining how much of one element is present in a compound (e.g., how much iron in Fe₂O₃).
- The ratio of parts (atoms of a specific element) to the whole (compound) can be applied to various compounds.
Counting Atoms and the Concept of the Mole
- Atoms are extremely small and impractical to count individually; mass measurement is used instead.
- Bulk items (like nails or atoms) are counted in groups (dozens or moles) for practicality.
- The mole (mol) is the chemist’s counting unit: 1 mol = 6.022 × 10²³ items (Avogadro’s number).
Moles, Mass, and Atomic Scale Calculations
- The number of items in a mole is constant, but the mass varies by substance.
- Molar mass (grams per mole) allows conversion between grams and moles using atomic or molecular mass from the periodic table.
- Avogadro’s number converts between moles and number of atoms/molecules.
Problem-Solving with Moles and Mass
- To convert grams to moles: use mass of sample and molar mass (g/mol).
- To convert moles to atoms: use Avogadro’s number as a conversion factor.
- To find molar mass of compounds: sum atomic masses for each element in the formula.
Parts-to-Whole Relationships in Compounds
- The ratio of individual atoms to a whole molecule (e.g., 2 O atoms per CO₂).
- Similar ratios apply for moles (e.g., 1 mol CO₂ contains 2 mol O).
- Conversion between mass of a compound and mass/amount of a specific element requires multiple steps (g→mol→ratios→g).
Key Terms & Definitions
- Mole (mol) — Unit for amount of substance; equals 6.022 × 10²³ entities.
- Avogadro’s Number — 6.022 × 10²³; number of particles per mole.
- Molar Mass — Mass (in g) of one mole of a substance; found on the periodic table.
- Parts-to-Whole Ratio — Proportion of specific atoms/ions in a compound relative to the whole.
Action Items / Next Steps
- Practice unit conversions using moles, grams, and Avogadro’s number.
- Learn to calculate molar mass for compounds from the periodic table.
- Review and practice parts-to-whole calculations for exam preparation.
- Ensure proficiency with scientific notation on your calculator.