Understanding Chemical Composition and Conversions

Oct 13, 2024

Chapter 6: Chemical Composition - Introductory Chemistry

Learning Objectives

  • Convert between moles and number of atoms, grams and moles, and grams and number of atoms or molecules.
  • Convert between moles of a compound and moles of a constituent element, and grams of a compound and grams of a constituent element.
  • Use mass percent composition as a conversion factor.
  • Determine mass percent composition from a chemical formula.
  • Derive empirical formula from experimental data and calculate molecular formula from empirical formula and molar mass.

Sodium and Dietary Considerations

  • Sodium is vital for fluid regulation but excess can cause high blood pressure.
  • FDA recommends 2.4 grams or 2400 milligrams of sodium per day.
  • Sodium chloride (NaCl) is table salt: 1 sodium ion to 1 chloride ion.
  • Mass differences between Na and Cl necessitate calculations for compound constituent elements.

Conversion Problems

  • Example: Nails by weight at a hardware store.
    • Use a solution map to convert pounds to nails.
    • Conversion factors: weight per dozen nails, number of nails per dozen.

The Mole and Avogadro's Number

  • A mole equals 6.022 x 10^23 particles, known as Avogadro's Number.
  • 1 mole of an element has a mass equivalent to its atomic mass in grams.
  • Allows counting atoms via weighing.

Converting Between Grams, Moles, and Atoms

  • Example: Convert between grams and moles using molar mass.
  • Example: Convert moles to atoms using Avogadro's number.
  • Example: Calculate mass of a compound in molecules or formula units.

Chemical Formulas as Conversion Factors

  • Use the chemical formula to convert between moles of compounds and constituent elements.
  • Example: Find moles of oxygen in calcium carbonate (CaCO3).

Mass Percent Composition

  • Mass percent of element X = (mass of X / mass of compound) x 100
  • Use mass percent composition as a conversion factor between grams of an element and grams of a compound.

Empirical Formulas

  • Empirical formula gives smallest whole-number ratio of elements in a compound.
  • Steps to determine empirical formula:
    1. Convert mass of each element to moles.
    2. Write pseudo formula using moles as subscripts.
    3. Divide by smallest subscript to get whole numbers.

Molecular Formulas

  • The molecular formula is a multiple of the empirical formula.
  • Use molecular mass to find the whole number multiple (n).
  • Calculate n = molar mass / empirical formula mass.

Ozone Layer and CFCs

  • CFCs destroy ozone; were banned in 1996 due to environmental impact.
  • Ozone protects Earth from UV radiation.

Summary

  • Moles and molar mass facilitate counting atoms and molecules by mass.
  • Chemical formulas provide insight into composition and proportion of elements within a compound.
  • Empirical formulas derived from experimental data can lead to determination of molecular formulas.

This chapter provides foundational skills in chemical composition, useful in various applications of chemistry including environmental science, health sciences, and materials science.