Honors Chemistry

Study Guide – Unit 15.2: Buffers & Titrations

Test Date: ______

A – The Common-Ion Effect

  • The extent of ionization of a weak acid or base is DECREASED by the addition of a common ion, according to Le Chatelier’s principle.
  • Adding a strong electrolyte to a weak acid causes the equilibrium to shift to the left, which decreases the [H+], causing the pH to increase (solution becomes more basic).
  • Adding a strong electrolyte to a weak base also causes the equilibrium to shift to the left, which decreases the [OH-], causing the pH to decrease (solution becomes more acidic).
  • An ICE table can be used to help you calculate the pH of a solution when a common ion is involved. Make sure that you can solve a problem of this type.

B – Buffered Solutions

  • Buffered solutions contain a WEAK conjugate acid-base pair that must:
  • Be particularly resistant to pH changes, even when a strong acid or base is added.
  • Contain an acidic species (the weak acid OR conjugate acid of a weak base) to neutralize the addition of any OH- ions and a basic species (conjugate base of the weak acid OR a weak base) to neutralize the addition of any H+ ions.
  • Not consume each other through a neutralization reaction.
  • A buffer which meets the criteria above is made by mixing a weak acid with a salt of that same weak acid (Ex: HC2H3O2 with NaC2H3O2) or a weak base with a salt of that same weak base (Ex: NH3 with NH4Cl)
  • A buffer’s capacity is the amount of acid or base the buffer can neutralize before the pH begins to change to an appreciable degree.
  • A buffer’s pH range is the range of pH values over which the buffer acts effectively.
  • The BEST buffer is one in which the moles of the weak acid are equal to the moles of its conjugate base, meaning that the pKa value is identical to the pH value.
  • The Henderson-Hasselbalch equation can be used to find the pH of a buffered solution. It can also be used to solve for the Ka value of the weak acid involved or to find the concentration of either the weak acid or its conjugate base. Make sure that you have this equation memorized and can solve for any of its components.

C – Titrations and Solution Stoichiometry

  • A titration is an experimental technique used to determine the concentration of a solution with an unknown molarity.
  • The solution of unknown concentration which is being titrated is referred to as the analyte. The solution of known concentration is referred to as the titrant or the standard solution.
  • The equivalence point of a titration is the point at which neutralization occurs and there are equal quantities of acid and base present in the solution.
  • The equivalence point does NOT have to occur at a pH = 7.
  • Acid-base indicators can be used to signal the end point of a titration by their color change. (Phenolphthalein is colorless in the presence of an acid, but pink in the presence of a base)
  • Be able to use either the molarity and volume of a standard solution or the grams of either the acid or base and a balanced neutralization reaction to find the concentration of the analyte.

D – Acid-Base Titrations & Titration Curves

  • A pH meter can be used to monitor the progress of a titration reaction.
  • When graphing a pH titration curve, pH appears on the y-axis and the volume of the added titrant (standard solution) appears on the x-axis.
  • A titration curve can be divided into four regions:
  • The initial pH: the pH of the analyte before the addition of any titrant
  • Between the initial pH and the equivalence point: as the titrant is added, the pH will slowly increase if the analyte is an acid, but slowly decrease if the analyte is a base
  • The equivalence point: here, the number of moles of acid and base are equal
  • After the equivalence point: the pH of the solution will continue to increase if the analyte is an acid, but decrease if the analyte is a base
  • Be able to recognize the following titration curves:
  • Titrating a strong acid (analyte) with a strong base (titrant) - very low initial pH, equivalence point at pH = 7, large rapid-rise portion of the curve
  • Titrating a strong base (analyte) with a strong acid (titrant)- very high initial pH, equivalence point at pH = 7, large rapid-rise portion of the curve
  • Titrating a weak acid (analyte) with a strong base (titrant) – initial pH below 7 but higher than that of a strong acid, equivalence point at pH > 7, small rapid-rise portion of the curve
  • Titrating a weak base (analyte) with a strong acid (titrant) – initial pH above 7 but lower than that of a strong base, equivalence point at pH < 7, small rapid-rise portion of the curve
  • As the Ka for an acid decreases (acid becomes weaker in strength), the pH at the equivalence point increases. Because the rapid-rise portion of the curve is smaller for a weak acid, the choice of indicator is more critical than it is for a strong acid.
  • When weak acids contain more than one ionizable H atom (polyprotic acid), each neutralization step is sufficiently separated, meaning that there are multiple equivalence points, one for each H atom available (ex: a diprotic acid would have 2 equivalence points).