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Chem 1B Worksheet 4: The Common Ion Effect and Buffer Chemistry
For questions and general help contact Dave Swanson at [email protected].
Learning Objectives
 Students will be able to name the conjugate of an acid or a base
 Students will be able to describe the Common Ion Effect
 Students will be able to articulate the purpose and contents of a buffer solution
 Students will be able to describe the fundamental workings of a buffer solution
Exercise 1: Buffer Chemistry: The Conjugate Pair and The Common Ion Effect
As reviewed in the previous worksheet, the dissociation of weak acids is subject to an
equilibrium constant Ka. For the weak acid HF,
HF + H2O ⇌ H3O+ + F- (Reaction 1)
Ka =
[H 3O+ ][F- ]
[HF]
a) Write the equation for the dissociation of the weak acid HCN in water.
b) Write the expression for the Ka of HCN.
c) Ka for HCN is 6.17 * 10-10. Given this figure, what can you say about the relative
amounts of the non-dissociated acid HCN and its conjugate base in the solution
(approximately; i.e., roughly equal, one much larger, etc.)?
Note the dissociation equation for HF above (Reaction 1). This reversible reaction can be
seen as a simple proton exchange. In the forward direction, the weak acid HF donates a
proton to H2O, forming the hydronium ion H3O+; in the reverse direction, the weak base
F- takes a proton from a water molecule, reforming the acid HF. HF and F- are thus just
partners in a proton exchange process. An acid/base pair with this relationship is called a
conjugate pair. F- is the “conjugate base” of HF, and HF is the “conjugate acid” of F- .
(proton)
-
+
HF
(proton)
H2O
F-
+
+
H3O
d) What is the conjugate base of HCN? Of HNO2?
Suppose that we add a quantity of the water-soluble ionic compound potassium cyanide
(KCN) to a solution of HCN.
e) What is the reaction between KCN and water? (Hint; review your solubility
tables.) What new chemical species (compounds/ions) appear in solution when
KCN is added?
f) Note the equilibrium expression for Ka for HCN (problem b above). How has the
balance of equilibrium been changed by the addition of KCN to the solution?
How will this affect the expression for Ka? What is the effect of these changes on
the pH of the solution?
The changes made by the addition of KCN above are called the “common ion effect”.
If we realize that the equilibrium constant Ka is constant, and cannot be changed, it must
be the case that the increase in concentration of of cyanide ion ([CN-]) to a solution forces
a change in the concentration of other solution components so as to bring the solution
back to equilibrium.
Exercise 2: Introduction to Buffer Solutions
A buffer solution is an aqueous mixture of chemical species which is designed to resist
changes in pH when acids or bases are added to it. A buffer solution contains both a
weakly acidic species (which will consume strong bases added to the solution) and a
weakly basic species (which will consume strong acids).
g) Buffer solutions are very important in the field of medicine. Can you think of a
good application for a buffer solution?
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A buffer solution must be designed so that the weak acid and weak base do not react with
each other; thus, buffers typically contain both a weak acid and its conjugate base. A
reaction between these substances results only in a proton transfer, with has no net effect
on the solution or its pH:
(proton)
-
+
HA
(proton)
A-
+
A-
HA
As a result, the weak acid and its conjugate base will coexist in the buffer solution,:
Buffer Solution
(Mix of Acid and Base)
Acetic acid (HCH3CO2) is a weak acid commonly used in buffer solutions. Acetic acid
belongs to the family of carboxylic acids, which contain an acidic carboxyl group with an
acidic proton. (In the diagram below, for acetic acid, R = CH3.)
carboxyl
group
O
R
OH
h) Write the equation for the dissociation reaction for acetic acid in water. What is
the conjugate base of acetic acid? Write the expression for the Ka of acetic acid.
i) The Ka of acetic acid is very small (1.8 * 10-5). What does this tell you about the
amount of the conjugate base which will exist in a solution of acetic acid? What
must be done to form a buffer solution from a solution of acetic acid?
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Exercise3: Buffer Chemistry
As stated above, a buffer solution resists changes in pH by consuming and countering the
effects of strong acids and bases. A brief overview of how a buffer is made and how it
serves to counteract changes in pH can be seen at the link below. (If this link does not
open, just double-click on the Powerpoint file posted with this worksheet.) Use the file in
Presentation Mode to see the animation.
Making and Using a Buffer
The buffer solution in in this powerpoint file is made of acetic acid (HCH3CO2) and
sodium acetate (NaCH3CO2) and is found very commonly in practice. The questions
below refer to this buffer.
j) What is the purpose of adding sodium acetate to the solution?
k) List all the compounds and ions which will be present in the solution at
equilibrium. Which of these are relevant to the key function of the buffer?
Which are not? Why?
Equal volumes of 1M acetic acid and 1M sodium acetate are mixed.
l) Roughly, what are the relative volumes of the compounds in solution (from
question k)? (Equal, greater, much greater, etc.)
m) Is this a buffer solution? Why or why not?
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n) Suppose that a quantity of the strong acid HNO3 is added to this solution. Which
of the compounds in the solution (see question k above) will react with this acid?
Write the equation for this reaction.
o) Given the reaction equation you have written above, can you draw any
conclusions about which way the reaction will progress (e.g., towards the right, or
to products, or to the left, or reactants)?
p) Suppose that a quantity of the strong base NaOH is added to this buffer. What
species in the in the solution will these ions react with? Write the equation for
this reaction.
q) Given the reaction equation you have written above, can you draw any
conclusions about which way the reaction will progress (e.g., towards the right, or
to products, or to the left, or reactants)?
Rev 7 10-1-10
5
Rev 0 7/19/10
Rev 1 7/20/10
 Added questions, changed order of questions
 Added Ex. 2
Rev 2 7/19/10
 More questions, revisions
 Added CIE exercise
Rev 3 7/25/10
 Rearranged Ex. 2 and 3
 More CIE questions
Rev 4 7/28/10
 Question rearrangement and clarification.
Rev 4/5 8/12/10
 Added buffer animation
 Added text to Ex 1
 Miscellaneous question clarification
Rev 6 8/15/10
 Added conjugate pair infor/q’s to ex. 1
 Sharpened up notes, questionsg
Rev 7 10/1/10
 Edited for posting
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