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Chem 1B Worksheet 3: Concepts of Acid/Base Equilibrium
Learning Objectives
 Students will be able to describe the difference between a strong and weak acid or base
 Students will be able to articulate the conceptual and mathematical relationships between pH,
Ka and Keq
 Students will be able to describe the solution of the equation for Ka given initial conditions
Exercise 1: Strong vs. Weak Acids and Bases
In Chem 1A, the study of acids and bases was confined primarily to “strong” acids and bases.
As you might recall, the terms “strong” and “weak” have specific meanings when applied to
acids and bases. The term “strong” implies that the dissociation of the acid or base in water goes
to virtual completion; for example,
Strong acid:
Strong base:
HCl + H2O  H3O+ + ClNAOH  Na+ + OH-
Notice that these reactions are written as one-way reactions.
a) Write the reaction of the strong acid HNO3 with water. What are the reactants and
products of this reaction? Is this a one-way or a reversible reaction? If 0.1 mol of HNO3
is added to beaker of water, how many moles of each of the products are produced?
b) In General Chemistry, there are only six acids which are considered to be “strong”; all
the rest are weak. What are these six acids?
As opposed to a strong acid or base, a weak acid or base will not completely dissociate in
aqueous solution. Only a portion of the acid (and often a very small portion) will be ionized,
leaving a quantity of the non-ionized acid remaining in solution in molecular form. Accordingly,
the reaction of a weak acid or base in aqueous solution is written as a reversible reaction. For
example:
Weak Acid: HF + H2O ⇌ H3O+ + FWeak Base: NH3 + H2O ⇌ NH4+ + OH-
Exercise 2: Aqueous Equilibria of Acids and Bases
The basic principles of acid/base equilibria are identical to those of general chemical equilibria.
Because the topic of acid/base equilibrium is so central to the science of chemistry, it has its own
unique terminology.
Since the reactions of weak acids and bases do not go to completion, they can be written as
reversible reactions. Accordingly, an equilibrium constant can be written for their dissociation in
water. For a weak acid in water (HA + H2O ⇌ H3O+ + A-),
K eq =
[H 3O+ ][A - ]
[HA][H 2O]
For acid dissociation, this equilibrium is called Ka, and since H2O is the solvent for this reaction
of aqueous reactants, it is left out of the equation. For example, for the dissociation in water of
the weak acid HCN,
[H O+ ][CN - ]
Ka = 3
[HCN]
For the dissociation of a weak base in water (B + H2O ⇌ BH+ + OH-), the term Kb is used:
Kb = [BH+] [OH-]
[B]
For example, for the weak base NH3, NH3 + H2O ⇌ NH4 + OH-, and
Kb =
[NH 4 + ][OH - ]
[NH 3 ]
a) Write the expression for Ka for the dissociation of the weak acid HF in water.
b) Ka for the weak acid HF is 7.2*10-6, and Ka for HCN is 4.0*10-10. Which will have a
higher [H3O+]? Explain in terms of the above equations.
c) Two solutions of the weak acid HF are found to have pH readings of 2.7 and 3.0
Which of these solutions has the highest equilibrium concentration of HA?
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Water can be said to undergo a dissociation reaction, forming hydronium and hydroxide ions.
2H2O ⇌ H3O+ + OH-
The dissociation of water thus has its own equilibrium constant. Because this constant is
uniquely important to acid/base chemistry, it is given its own symbol, Kw. Its value at 25°C is
Kw = [H3O+][OH-] = 1.0 * 10-14
d) What are the concentrations of hydronium (H3O+) and hydroxyl (OH-) ions in pure
water?
e) What is the pH of pure water?
f) A solution is found to have a pH of 3.0. What is the [OH-] for this solution?
Exercise 3: Relationship Between Ka, pH, and Concentration
In describing the properties of an acidic solution, the terms concentration, pH, and Ka are all
interrelated. This relationship can be observed in a plot of these parameters for any weak acid.
Below is a plot of pH vs. concentration for solutions of acetic acid at equilibrium:
HCH3COO + H2O ⇌ H3O+ + CH3COO- (Reaction 1)
Concentration vs pH For Acetic Acid
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2.9
2.8
pH
2.7
2.6
2.5
2.4
2.3
2.2
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
Concentration of Acetic Acid
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a) Write the expression for Ka of acetic.
b) Write the general expression for pH. What does pH measure?
c) From the plot above, what is [H3O+] when [HA] = 0.9M (best approximation)?
d) Describe the relationship between the data on the two axes of the above plot. Why does
this curve slope down to the right?
e) Write the equation for Ka for the acid in the plot (acetic acid).
f) Using data from the plot, calculate your best estimate for the value of Ka of a 0.2M
solution of acetic acid. (Use a value of pH to a precision of 0.01 from the plot above.)
g) What would you expect if you were to calculate Ka at M = 0.5? Would your calculated
value be lower, higher, or equal to your calculation in the previous question? Explain
your answer.
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The following questions refer to the plot below, which shows the relationship between pH and
acid concentration for several weak acids.
Molarity vs pH For Some Weak Acids
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Nitrous acid
2.9
Formic acid
2.8
Propanoic acid
2.7
Acetic acid
2.6
2.5
pH=2.37
pH
2.4
2.3
pH=2.36
pH=2.37
2.2
2.1
2
1.9
1.8
1.7
1.6
1.5
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
Concentration of Acid (M)
h) Which is the weakest acid on the above chart? Which is the strongest? Which will have
the highest and lowest Ka? Why?
i) At a concentration of 0.5M, which acid will have the highest concentration of hydrogen
ions? The lowest? Why?
j) For each acid, why does the pH decrease as the concentration increases?
k) Note the three circled data points on the plot above; they signify points at which the pH
values for three different acids are approximately equal. Why are the concentrations at
these three points so different?
l) Calculate the value of Ka for each of the three weak acids in the question above. Do they
values you calculate make sense in terms of your answers to the above questions?
Rev 11 9/22/10
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Revision History
Rev 3
2/11/10
 Added Learning Objectives
 Streamlined Exercise 2
o Removed details and non-conceptual questions
o Added drawing questions
o Added single-curve plot to work on understanding of curve
Rev 4
3/01/10
 Major overhaul; corrected equations, etc.
Rev 6
7/14/10
 Changed single-line plot and following question
 Minor question clarifications and typo fixes
Rev 7
7/19/10
 General question clarifications and typo fixes
Rev 8
7/28/10
 General question clarifications
Rev 9
7/29/10
 Added water chemistry questions
Rev 10
8/14/10
 Removed redundant water question
 Clarified plot questions
Rev 11
9/22/10
 Clean up for posting
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