18.2 Calculations involving acids and bases

The strengths of acids and the acid dissociation constant

  • Strong acids and bases:
    • Since strong acids are assumed to completely dissociate in water, the concentration of each of the two ions produced is the same as the initial concentration of a strong monoprotic acid (one hydrogen)

Strong acid concentration

  • Weak acids and bases:
    • Since these do not complete dissociate, the concentration of products is not the same
    • The way to solve for concentrations is using the equilibrium expression
    • Acid dissociation constant:
      • ka
    • Base dissociation constant:
      • kb

Calculating Ka and Kb

  1. Write out the complete reaction
  2. Complete an ICE table
  3. Determine Ka or Kb

Ka and Kb for a conjugate acid-base pair

  • kw
  • Strong acid
    • Large Ka
    • Weaker conjugate base
    • Smaller Kb of conjugate base
  • Strong base
    • Large Kb
    • Weaker conjugate acid
    • Smaller Ka of conjugate acid

The temperature dependence of Kw

  • Kw is defined only at 25°C
  • The ionization of water is endothermic, so as temperature increases, equilibrium shifts right
  • Increase in concentration of H+ results in decrease in pH

pKa and pKb

  • The pH scale can be applied to Ka and Kb to get pKa and pKb



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