AP Chemistry
Chapter 16. Acid-Base Equilibria
Chapter 16. Acid-Base Equilibria
Sample Exercise 16.10 (p. 682)
A student prepared a 0.10 M solution of formic acid (HCHO2) and measured its pH. The pH at 25oC was found
to be 2.38. Calculate the Ka for formic acid at this temperature. (1.8 x 10-4)
Practice Exercise 16.10
Niacin, one of the B vitamins, has the following molecular structure:
A 0.020 M solution of niacin has a pH of 3.26.
What is the acid-dissociation constant, Ka, for niacin?
(1.5 x 10-5)
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AP Chemistry
Chapter 16. Acid-Base Equilibria
Sample Exercise 16.11 (p. 684)
A 0.10 M solution of formic acid (HCOOH) contains 4.2 x 10-3 M H+(aq). Calculate the percentage of the acid
that is ionized.
(4.2%)
Practice Exercise 16.11
A 0.020 M solution of niacin has a pH of 3.26. Calculate the percent ionization of the niacin.
(2.7%)
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AP Chemistry
Chapter 16. Acid-Base Equilibria
Using Ka to Calculate pH
•
Using Ka, we can calculate the concentration of H+ (and hence the pH).
1. Write the balanced chemical equation clearly showing the equilibrium.
CH3COOH(aq)  H+(aq) + CH3COO-(aq)
2. Write the equilibrium expression. Look up the value for Ka (in a table).
Ka = [H+][ CH3COO-] = 1.8 x 10-5
[CH3COOH]
3. Write down the initial and equilibrium concentrations for everything except pure water.
• We usually assume that the equilibrium concentration of H+ is x.
4. Substitute into the equilibrium constant expression and solve.
Ka = 1.8 x 10-5 = [H+][C2H3O2-] = (x)(x)
[HC2H3O2]
0.30 - x
Note that Ka is very small (1.8 x 10-5) relative to [HC2H3O2] (0.30 M).
Keep this x

x2 = 1.8 x 10-5
0.30 – x
Neglect x in the denominator since it is extremely small relative to 0.30.
(Use ballpark figure of 103 X difference for neglecting x in the denominator )

x2 = (1.8 x 10-5 )(0.30) = 5.4 x 10-6
  x2 = 5.4 x 10-6)
X = 2.3 x 10-3 M = [H+]
Check: Compare x with original [HC2H3O2] of 0.30 M:
2.3 x 10-3 M x 100% = 0.77%, which is < 5%
0.30 M
5. Convert x ([H+]) to pH.
pH = - log (2.3 x 10-3) = 2.64
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AP Chemistry
Chapter 16. Acid-Base Equilibria
Sample Exercise 16.12 (p. 685)
Calculate the pH of a 0.20 M solution of HCN. Refer to Table 16.2 for Ka. (5.00)
Practice Exercise 16.12
The Ka for niacin is 1.6 x 10-5. What is the pH of a 0.010 M solution of niacin? (3.41)
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AP Chemistry
Chapter 16. Acid-Base Equilibria
Sample Exercise 16.13 (p. 687)
Calculate the percentage of HF molecules ionized in
a) a 0.10 M HF solution (7.9%)
b) a 0.010 M HF solution (23%)
Practice Exercise 16.13
In Practice Exercise 16.11, we found that the percent ionization of niacin (Ka = 1.5 x 10-5) in a 0.020 M solution
is 2.7%. Calculate the percentage of niacin molecules ionized in a solution that is
a) 0.010 M
(3.9%)
b) a 1.0 x 10-3 M (12%)
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AP Chemistry
Chapter 16. Acid-Base Equilibria
Sample Exercise 16.14 (p. 689)
The solubility of CO2 in pure water at 25oC and 0.1 atm pressure is 0.0037 M. the common practice is to assume
that all of the dissolved CO2 is in the form of carbonic acid (H2CO3), which is produced by reaction between the
CO2 and H2O:
CO2(aq) + H2O(l)  H2CO3(aq)
What is the pH of a 0.0037 M solution of H2CO3?
(4.40)
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AP Chemistry
Chapter 16. Acid-Base Equilibria
Practice Exercise 16.14 (p. 690)
a) Calculate the pH of a 0.020 M solution of oxalic acid (H2C2O4). (see Table 16.3 for Ka1 and Ka2).
b) Calculate the concentration of oxalate ion [C2O42-].
(pH = 1.80; [C2O42-] = 6.4 x 10-5 M)
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AP Chemistry
Chapter 16. Acid-Base Equilibria
Sample Problem 16.15 (p. 691)
Calculate the concentration of OH- in a 0.15M solution of NH3.
(1.6 x 10-3 M)
Practice Problem 16.15
Which of the following compounds should produce the highest pH as a 0.05 M solution: pyridine, methylamine,
or nitrous acid?
(methylamine)
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AP Chemistry
Chapter 16. Acid-Base Equilibria
Sample Exercise 16.16 (p. 692)
A solution made by adding solid sodium hypochlorite (NaClO) to enough water to make 2.00 L of solution has a
pH of 10.50. How many moles of NaClO were added to the water? (See info immediately above.)
(0.60 mol)
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AP Chemistry
Chapter 16. Acid-Base Equilibria
Practice Exercise 16.16
A solution of NH3 in water has a pH of 11.17. What is the molarity of the solution?
(0.12 M)
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AP Chemistry
Chapter 16. Acid-Base Equilibria
Sample Exercise 16.17 (p. 695)
Calculate
a) the base-dissociation constant, Kb, for the fluoride ion (F-); (1.5 x 10-11)
b) the acid-dissociation constant, Ka, for the ammonium ion (NH4+). (5.6 x 10-10)
Practice Exercise 16.17
a) Which of the following anions has the largest base-dissociation constant: NO2-, PO43-, or N3-?
(PO43-, Kb = 2.4 x 10-2)
b) The base quinoline has the following structure:
Its conjugate acid is listed in handbooks as having a pKa of 4.90.
What is the base-dissociation constant for quinoline?
(7.9 x 10-10)
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AP Chemistry
Chapter 16. Acid-Base Equilibria
Sample Exercise 16.18 (p. 698)
Determine whether aqueous solutions of each of the following salts will be acidic, basic, or neutral:
a) Ba(CH3COO)2,
b) NH4Cl
c) CH3NH3Br
d) KNO3
e) Al(ClO4)3
Practice Exercise 16.18
In each of the following, indicate which salt in each of the following pairs will form the more acidic (or less
basic) 0.010 M solution:
a) NaNO3 or Fe(NO3)3
b) KBr, or KBrO
c) CH3NH3Cl or BaCl2
d) NH4NO2 or NH4NO3
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AP Chemistry
Chapter 16. Acid-Base Equilibria
Sample Exercise 16.19 (p. 698)
Predict whether the salt Na2HPO4 will form an acidic or a basic solution on dissolving in water.
(basic)
Practice Exercise 16.19
Predict whether the dipotassium salt of citric acid (K2HC6H5O7) will form an acidic or basic solution in water.
(see Table 16.3 for data)
(acidic)
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AP Chemistry
Chapter 16. Acid-Base Equilibria
Sample Exercise 16.20 (p.702)
Arrange the compounds in each of the following series in order of increasing acid strength:
a) AsH3, HI, NaH, H2O;
b) H2SO4, H2SeO3, H2SeO4.
Practice Exercise 16.20
In each of the following pairs choose the compound that leads to the more acidic (or less basic) solution:
a) HBr, HF;
b) PH3, H2S;
c) HNO2, HNO3;
d) H2SO3, H2SeO3.
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AP Chemistry
Chapter 16. Acid-Base Equilibria
Sample Integrative Exercise 16 (p. 706)
Phosphorous acid (H3PO3) has the following Lewis structure:
a) Explain why phosphorous acid is diprotic and not triprotic.
b) A 25.0 mL sample of a solution of H3PO3 is titrated with 0.102 M NaOH. It requires 23.3 mL of NaOH to
neutralize both acidic protons. What is the molarity of the H3PO3 solution?
c) This solution has a pH of 1.59. Calculate the percent ionization and Ka1 for H3PO3, assuming that
Ka1 >>Ka2.
d) How does the osmotic pressure of a 0.050 M solution of HCl compare qualitatively with that of a 0.050 M
solution of H3PO3? Explain.
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