Worksheet 10 – Chapter 18 – Common Ion Effect and Buffers KEY
I. Common Ions
A. When a weak acid solution has common ions added by a salt, the equilibrium will shift by Le
Chatelier's principle. The shift in equilibrium position that occurs because of the addition of an
ion already involved in the equilibrium reaction is called the common ion effect.
B. A buffered solution is one that resists a change in its pH when either hydroxide ions or
protons are added. Buffered solutions are simply solutions of weak acids or bases containing a
common ion. When hydroxide ions are added to a buffer solution they react with the acid and are
replaced by the Anions.
C. If you know the ratio of acid to base and both amounts are large the proton concentration can
easily be found using the equation:
To find the pH easily the Henderson-Hasselbalch equation can be used:
D. Buffer capacity of a buffered solution represents the amount of protons or hydroxide ions the
buffer can absorb without a significant change in pH. Most effective buffer solutions contain
large amounts of acid and conjugate base.
E. Titrations are commonly used to determine the amount of acid or base in a solution. The
progress of an acid-base titration is often monitored by plotting the pH of the solution being
analyzed as a function of the amount of titrant added. This is called a pH curve. The optimum
area for buffering in a pH curve is in the vicinity where pKa equals the pH (or where the ratio of
A- to HA equals 1). The equivalence point is where the amount of protons is equal to the amount
of hydroxide ions.
F. Indicators are actually weak acids themselves that change color when H
is added or taken
away from the structure. The best indicator for a titration is given by a indicator with pK
endpoint closest to the pH of the equivalence point.