Summary of 8th chapter from Chemical Principles: Zumdahl and Decoste. Notes containing key concepts from the chapter and thorough explanations of the terminology. Also includes formulas and relevant course-related information.
Applications of Aqueous
equilibria
8.1 Solutions of Acids or Bases containing a common ion.
The common ion is one that is being produced by a salt and an acid/base at the
same time within a solution.
Common ion effect consists of creating stress in a reaction and changing the
equilibrium position thru the production of an ion that is product in 2 different
reactions. Such that the product from first reaction causes the second reaction to
tend backward (which has the same product), Lé Châtelier’s principle.
Common ion effect also affects polyprotic dissociation.
8.2 Buffered Solutions
A buffered solution is one that resists a change in pH even when either hydroxide
ions or protons are added.
Blood is an example of a buffered solution.
A buffered solution contains a weak acid and its salt or a weak base and its salt.
Ka = [H+] [A-]/[HA]
-log[H+] = -log (Ka) – log ([HA]/[A-]) Henderson Hasselbalch equation
In buffer solutions, the pH depends upon the ratio of [HA]/[A -].
When H+ is added to buffered solutions, it reacts essentially with weak base present.
When OH- is added to a buffered solution, it reacts essentially with the weak acid
present.
8.3 Exact treatment of buffered solutions
, 8.4 Buffer capacity
Buffer capacity of a buffer solution is defined as the number of protons or hydroxide
it can absorb without significant change in pH.
The capacity of a buffer solution is determined by the magnitudes of [A -] and [HA]
The closer the ratio of [A-] and [HA], the higher the buffering capacity of a solution.
The pKa for the weak acid selected must be as close to the desired pH.
8.5 Titration and pH curves
Progress of an acid-base titration is monitored by plotting pH of the solution being
analyzed as a function of amount of titrant added; such function is called a pH curve.
Millimole is the unit used for titration calculations, equivalent to 1mol/1000
Molarity = millimole of solute/ mL of solution
For titration of strong acids with strong bases:
- Identify the amount of base that has reacted with H+ to produce H2O
- The remaining H+ will determine the pH.
- In case there is no more H+ and there exists an excess of OH- find the pH from
pOH or [H+] [OH-]
In performing reactions between weak acids and strong bases, it is important to
remember that even though the acid is weak, it reacts essentially to completion with
hydroxide ion, a very strong base.
For titration of weak acids with strong bases:
- Identify the amount of acid that has reacted with OH- to produce H2O, through
stoichiometry.
- The remaining acid will dissociate and produce H+, which will be used to find pH.
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