Chemistry: Stoichiometry, Atomic Mass, and Moles

Introduction to Chemistry

• Chemistry explains the visible world by describing the invisible components (atoms, molecules).
• Challenges in understanding due to our comparatively massive size.
• Focus on mass to understand, predict, and measure substances.

Importance of Measuring Mass

• Example: Atomic structure of sugar (C12H22O11) but practical use requires measuring mass.
• Stoichiometry: Science of measuring chemicals in reactions.
  • Greek origin: "measuring elements"
  • Allows counting atoms/molecules by weighing them.
• Stoichiometry's importance in translating microscopic to macroscopic quantities.

Measuring Elements

• Relative Atomic Mass: Average atomic mass of natural isotopes.
  • Example: Carbon isotopes (C-12, C-13, C-14).
  • Weighted average: 12.01 (based on C-12 standard).

Atomic Mass Units (AMU)

• Unit Selection: Arbitrary standard (similar to kilogram).
• 1 AMU: 1/12th of the mass of a carbon-12 atom.

Historical Evolution

• Initially used hydrogen, later oxygen, finally settled on carbon-12 in 1961.

Moles: Measuring Amount of Substance

• Mole: Translates atomic mass to grams.
• Definition: 6.022 x 10^23 atoms of carbon-12 = 12 grams.
  • Known as Avogadro's Number.
• Use to compare amounts of different substances.
  • Example: 1 mole of any element contains 6.022 x 10^23 atoms.
  • Converts atomic mass to molar mass.

Molar Mass Calculations

• Element Molar Mass: Same as relative atomic mass in grams (e.g., H = 1.008g, Fe = 55.85g).
• Compound Molar Mass: Sum of element molar masses.
  • Example: Sucrose (C12H22O11) molar mass calculation.

Chemical Reactions & Equations

• Equation Balancing: Ensuring equal atom numbers on both sides.
  • Follows Conservation of Mass.
• Balancing Steps:
  1. Start with the most complex molecule (e.g., sucrose).
  2. Balance carbon by assigning CO2.
  3. Balance hydrogen by assigning H2O.
  4. Balance oxygen last using O2.

Example Reaction: Sugar Metabolism

• Reaction: Sucrose + Oxygen -> Energy + CO2 + H2O.
• Balancing results: 12 CO2 + 11 H2O from one sucrose + O2.

Practical Stoichiometry: Quantifying Reactions

• Steps to calculate needed oxygen to burn a specific amount of sugar:
  1. Convert equation to molar masses.
  2. Compare ratios with the reactants in the experiment.
  3. Solve for unknown quantities.
• Example: Burning 5 grams of sugar requires 5.6 grams of oxygen.

Summary

• Key Units: Atomic mass units (AMU), Moles.
• Calculations: Molar mass, balancing equations.
• Practical Use: Molar ratios for predicting reaction amounts.