Lecture Notes on Measurements in Chemistry

Introduction to Mole Concept

• Measurements are crucial in science.
• The mole is a unit of measurement defined by Avogadro's number.
• Avogadro's number: (6.02 \times 10^{23}).
  • Comparable to counting a dozen (12) of something.
  • A large number designed to count elementary entities (atoms, molecules, ions, electrons).

Conversion Using Avogadro's Number

• Avogadro's number relates particles and moles.
• Example: For 13.3 moles of water, multiply by Avogadro's number:
  • (13.3 \times 6.02 \times 10^{23} = 8.66 \times 10^{24}) molecules.

Using Mass to Quantify Chemicals

• Relative Atomic Mass: Found on the periodic table.
  • Represents the mass of a single atom relative to (\frac{1}{12}) of the mass of a carbon atom.
  • No units since they cancel out through ratio comparison.
• Relative Formula Mass: Similar to atomic mass but for compounds.
  • Example: CO₂, calculated by summing atomic masses of constituent atoms.
  • (12.01 + 2 \times 16 = 44.01) units for CO₂.

Molar Mass

• Defined as the mass per mole of an element or compound (g/mol).
• Allows conversion between mass and moles using the periodic table values.
  • Example: To find the mass of 0.5 moles of NaCl:
    • Sodium = 22.99 g/mol
    • Chlorine = 35.45 g/mol
    • NaCl molar mass = 58.44 g/mol
    • (0.5 \times 58.44 = 29.2) grams of NaCl.

Converting Between Mass and Particles

• To convert from grams to particles:
  1. Use molar mass to convert grams to moles.
  2. Use Avogadro's number to convert moles to particles.
• Example: Convert 10.0 g of KBr to particles:
  • Molar mass of KBr = 119.2 g/mol
  • (10.0 \div 119.2 = 0.084) moles
  • (0.084 \times 6.02 \times 10^{23} = 5.06 \times 10^{22}) particles

Summary

• Avogadro's number and molar mass are essential conversion factors in chemistry.
• They allow conversions between particles, moles, and mass.
• Understanding and applying these concepts is critical in chemistry calculations.