The Common Ion Effect
1.
Calculate the molar solubility of silver thiocyanate (AgSCN; Ksp = 1.0 x 10 –12 ) in pure water and in water containing 0.01 M NaSCN.
2.
Calculate the pH of a solution containing 0.085 M nitrous acid (HNO
2
; K a
= 4.5 x 10 -4 ) and 0.1 M potassium nitrite.
3.
Consider the following equilibrium system: PbCl
2
(s)  Pb 2+ (aq) + 2Cl (aq) Describe what happens to the solubility of PbCl
2
when the following substances are added to the solution. Why? a.
Pb(NO
3
)
2 b.
NaCl c.
H
2
O d.
AgNO
3 e.
NaBr
Buffers
4.
Identify which of the following mixed systems could function as a buffer solution. For each system that can function as a buffer, write the equilibrium equation for the conjugate acid/base pair in the buffer system. a.
KF/HF b.
NH
3
/NH
4
Br c.
KNO
3
/HNO
3 d.
Na
2
CO
3
/NaHCO
3
5.
What is the pH of a 1 L solution containing 0.240 mol HC
2
H
3
O
2
and 0.180 mol NaC
2
H
3
O
2
? K a
(HC
2
H
3
O
2
) = 1.8 x
10 -5 .
6.
A buffer solution contains 0.384 M H
2
CO
3
and 0.239 M NaHCO
3
. If 0.0464 moles of potassium hydroxide are added to 225.0 mL of this buffer, what is the pH of the resulting solution? The K a
= 4.45 x 10 -7 . (Assume that the volume does not change upon adding potassium hydroxide.)

7.
A buffer solution contains 0.348 M ammonium chloride and 0.339 M ammonia. If 0.0248 moles of hydrochloric acid are added to 125.0 mL of this buffer, what is the pH of the resulting solution? The K a
of NH
4+
= 5.65 x 10 -10 .
(Assume that the volume does not change upon adding hydrochloric acid.)
8.
How many mL of 4.50 M sodium hydroxide must be added to 250.0 mL of a 0.200 M acetic acid solution to make a buffer with pH = 5.000? The K a
of acetic acid is 1.8 x 10 -5 . (See problem #5.)
Titrations
C
6
H
5
NH
2
(aq) + H
2
O(l)  C
6
H
5
NH
3
+ (aq) + OH – (aq)
9.
Aniline, a weak base, reacts with water according to reaction represented above. a.
Write the equilibrium expression, Kb, for the reaction represented above. b.
A sample of aniline is dissolved in water to produce 25.0 mL of a 0.10 M solution. The pH of the solution is 8.82. Calculate the equilibrium constant, K b
, for this reaction. c.
The solution prepared in part (b) is titrated with 0.10 M HCl. Calculate the pH of the solution when 5.0 mL of the acid has been added. d.
Calculate the pH at the equivalence point of the titration in part (c). e.
The pKa values for several indicators are given below. Which of the following indicators listed is most suitable for this titration? Justify your answer.
Indicator
Erythrosine
Litmus
Thymolphthalein pKa
3
7
10