Ex4: pH Chemistry: Acid Base Titrations and Buffers

• pH

  The measure of the acidity or alkalinity of solution
• pH = -log[H+]
• What is the difference between a strong acid and a weak acid?

  A strong acid gives up protons more readily than a weak acid
• The ______________ the acid, the ________________ the conjugate base.

  stronger; weaker ORweaker; stronger
• The equilibrium constant for a weak acid in water

  Ionization constant, Ka
• A solution that resists changes in pH

  A buffer
• the concentration ratio [base]/[acid] AKA [A-] / [HA] is also known as the

  buffer ratio
• The number of moles of A- and HA can be substituted for concentrations
• buffer capacity

  The number of moles of strong acid or strong base needed to change the pH of 1L of buffer by 1 unit
• A buffer's ability to resist a change in pH _____________ with _______________________ concentrations of conjugate acid and base.

  increases; increasing
• For the titration of a weak acid with a strong base, the indicator used is _________________

  phenolphthalein
• For the titration of a weak base with a strong acid, the indicator used is __________________

  methyl red
• end-point

  colour change is observed
• equivalence point


  the point at which the two solutions used in a titration are present in chemically equivalent amounts. pH point will increase rapidly when approaching this point
• half neutralization point

  pH is equal to pKa

  period before the equivalence point
• indicator transition range

  the rapid decrease of pH on the graph (steep portion)
• Ka (Acid Dissociation Constant)

  HA ⇌ H⁺ + A⁻

  Ka​=[H+][A−]​ / [HA]
• Kb (Base Dissociation Constant)

  B + H₂O ⇌ BH⁺ + OH⁻

  Kb​=[BH+][OH−]​ / [B]
• Kw (Ionization Constant of Water):

  H₂O ⇌ H⁺ + OH⁻

  Kw​=[H+][OH−] = 1.0×10−14
• pKa and pKb:

  negative logarithms of Ka and Kb

  pKa=−log(Ka​)pKb=−log(Kb​)
• pKw:

  pKw=−log(1.0×10−14)
• pKa + pKb = pKw = 14
• Ka × Kb = Kw = 1.0×10−14
• pH of a weak acid or weak base:

  • pH = -log [H+]
  • pOH = -log [OH-]
• pH of a buffer soln:

  • Henderson-Hasselbalch
  • pH = pKa + log [A-]/[HA
• buffer capacity

  • pH = pKa
• Ka and Kb equations are used when

  solving for ionization constants to then calculate for Henderson-Hasselbalch eq
• When selecting an indicator for acid-base titrations, choose an indicator whose pH range falls within the pH change of the reaction. For example, in the titration of a strong acid with a strong base, the pH quickly changes from 3 to 11