amount of at least one of the substances is determined. Based on the change in this one sub-
stance and the stoichiometry, the amounts of the other materials may be calculated.
Measurements may include the pressure, the mass (to be converted to moles), the
volume (to be used in calculations), and the pH (to be converted into either the hydrogen
ion or hydroxide ion concentration). Some experiments measure the color intensity (with
a spectrophotometer), which may be converted to a concentration.
Do not make the mistake of “measuring” a change. Changes are never measured; they
are always calculated.Common Mistakes to Avoid
1. Be sure to check the units and significant figures of your final answer.
2. When writing equilibrium constant expressions, use products over reactants. Each con-
centration is raised to the power of the coefficient in the balanced chemical equation.
3. In converting from Kcto Kpbe sure to use the ideal gas constant, R, whose units are
consistent with the units of the partial pressures of the gases.
4. Remember, in working Le Cha^telier problems, pressure effects are important only for
gases that are involved in the equilibrium.
5. Be sure, when working weak-base problems, to use Kband not Ka.
6. In titration problems, make sure you compensate for dilution when mixing two solu-
tions together.- A Kaexpression must have [H+] in the numerator, and a Kbexpression must have
[OH−] in the numerator.
Review Questions
Answer the following questions in 35 minutes. You may not use a calculator. You may use
the periodic table at the back of the book.Equilibrium 231- A 0.1-molar solution of acetic acid (CH 3 COOH)
has a pH of about
(A) 1
(B) 3
(C) 7
(D) 10
(E) 14 - Acid Ka, acid dissociation constant
H 3 PO 4 7.2 × 10 −^3
H 2 PO 4 − 6.3 × 10 −^8
HPO 42 − 4.2 × 10 −^13
Using the above information, choose the best
answer for preparing a pH =8 buffer.
(A) K 2 HPO 4 +KH 2 PO 4
(B) H 3 PO 4
(C) K 2 HPO 4 +K 3 PO 4
(D) K 3 PO 4
(E) K 2 HPO 4 +H 3 PO 4- Ka, the acid dissociation constant, for an acid is
9 × 10 −^4 at room temperature. At this tempera-
ture, what is the approximate percent dissocia-
tion of the acid in a 1.0 M solution?
(A) 0.03%
(B) 0.09%
(C) 3%
(D) 5%
(E) 9%
Use the following information for questions 4–6.
(A) a solution with pH =7.
(B) a solution with a pH <7, which is not a
buffer
(C) a solution with a pH <7, which is a buffer
(D) a solution with a pH >7, which is not a
buffer
(E) a solution with a pH >7, which is a bufferTIP