3/6/14 We went over these in class right before the exam and here is the summary
Correction for Practice Exam II (chapters 14-15)
21. The Kb for NH3 at 25oC is 1.8×10-5. What’s the pOH for 0.4 M NH4NO3 aqueous solution?
(A) 2.57
(B) 4.98
(C) 5.44
(D) 9.17
(E) 13.95
Hint: For simplicity, we do not consider the auto-ionization of water. The pH = 4.83.
Further Explanation for Practice Exam II (chapters 14-15)
17. Which of the following lies predominately to the right upon equilibrium?
(A) SO42- (aq) + HCO3- (aq)  HSO4- (aq) + CO32-(aq)
(B) NO2- (aq) + HCO3- (aq)  HNO2 (aq) + CO32-(aq)
(C) CH3COOH(aq) + HCOO- (aq)  CH3COO- (aq) + HCOOH(aq)
(D) HCOOH (aq) + CN-(aq)  HCOO- (aq) + HCN (aq)
(E) HCN (aq) + F-(aq)  CN- (aq) + HF (aq)
Hint: The reaction direction for acid and base is always from
“stronger acid + stronger base weaker conjugate base + weaker conjugate acid”
When making comparison, we compare the acid strength between the acid and the conjugate
acid; the basic strength between the base and the conjugate base. All are in aqueous solutions.
Use what you learned from chapter 4 of strong/weak acid/base concept from Arrhenius acid and
base definition and the acidic/basic strength from chapter 15 concerning the conjugate acid and
base to determine the equilibrium direction. In the exam, the Ka and Kb usually are not given unless
the question involves calculation. If the Ka and Kb are available, some requiring calculation from the
relationship of Ka x Kb = Kw, then we compare the Ka of the acid and the Ka of the conjugate acid; and
the Kb of the base and the Kb of the conjugate base.
For (A), the SO42- (aq) is the base for it accepts proton from hydrogen carbonate and HCO 3- (aq) is
the acid. Thus, HSO4- (aq) is the conjugate acid of SO42- (aq) and CO32-(aq) is the conjugate base of
HCO3- (aq). So we compare the basic strength between SO42- (aq) and CO32- (aq); or compare the
acid strength between HCO3- (aq) and HSO4- (aq). Since SO42- (aq) is from strong acid, sulfuric acid,
and thus it is neutral; CO32- (aq) is from weak acid, carbonic acid, and thus it is a weak base. So the
reaction direction is from the right to left.
For (B), NO2- (aq) is the base and HCO3- (aq) is the acid; HNO2 (aq) is the conjugate acid and CO32-(aq)
is the conjugate base. Comparing the acid strength between HCO3- (aq) and HNO2 (aq) is easier than
comparing the basic strength between NO2- (aq) and CO32-(aq). According to chapter 4 and chapter
15 HNO2 (aq) is a weak acid and HCO3- (aq) is the conjugate base of H2CO3 (aq), a weak acid. Thus,
according to chapter 15, CO32-(aq), obtaining by removing the second hydrogen ion, is a weak base
but it is a much stronger weak base than NO2- (aq), obtaining by removing the first hydrogen ion.
For (C), in chapter 2 carboxylic acid, RCOOH, and chapter 9 and chapter 10 talking about the polarity
of bond and molecule. Thus, more C on the R, weaker the acidity assuming that all the surrounding
atoms on R are hydrogens. For (D), consider the bond polarity and the cumulative effect on
molecular polarity.