Practice problems for 15
1. Give the equilibrium as PO43- (aq) + H2O(l)  HPO42- (aq) + OH- (aq)
(a) Identify the conjugate acid-base pairs. PO43- and HPO42- ; H2O and OH- (Example
15.1)
(b) What are the conjugate acid and conjugate base respectively for OH-? Conjugate
acid: H2O; conjugate base: O2-. (p. 646)
2. Which of the following statements is true with respect to the reaction below?
A)
B)
C)
D)
The reaction favours the formation of CH3COO–.
CH3COO– is a weaker base than Cl–.
Cl– is a weaker base than CH3COO–.
The reaction favours the formation of HCl.
3. Calculate the pH for the following solutions: (a) 2.5 x 10–2 M HNO3(aq) 1.60
(b) 2.5 x 10–2 M NaOH (aq) 12.40 (c) 4 x 10–10 M HNO3(aq) 7.00 (d) 4 x 10–10 M NaOH
(aq) 7.00
Hint: this is a question about strong acids and bases. There are two sources of H+
and OH-. When concentration is very small, the contribution from water can not be
ignored.
4. A solution of HNO3 has a pH of 3.5. (a) What is its OH- concentration in molarity?
3.16x10-11 M , pOH=14-pH=10.5, [OH-]=10-pOH=10-10.5
(b) How many grams of HNO3 are there in 300 mL of this solution?
0.006 grams
[H+]=10-pH=10-3.5=3.16x10-4M,
n=MxV=3.16x10-4Mole/L x 0.3L=9.49x10-5mol,
m=nxM= 9.49x10-5molx63g/mol=0.006g
4. A solution was made by dissolving 2.5 grams Ca(OH)2 in 200 mL final volume.
(Example 15.6)
(a) What is the molar concentration of OH- in the solution? 0.33 M (b) What is its pH?
13.52
5. The Kb for dimethylamine, (CH3)2NH, at 25oC is 9.6×10-4. What’s the Ka of its
conjugate acid?
(A) 9.6×10-3 (B) 9.6×10-4 (C) 1.04×10-10 (D) 1.04×10-11 (E) 1.0×10-14
Hint: KaKb = Kw
6.
A)
B)
C)
D)
The pH of a 0.1 M aqueous NH3 solution (Kb = 1.8x 10–5) will be _____.
2.89
11.13
3.89
10.11
7. A solution of acetic acid has a pH of 3.45. What is the concentration of acetic acid in
this solution? Ka for CH3COOH is 1.8×10-5.
(A) 3.55×10-8 (B) 1.80×10-5
(C) 6.99×10-3 (D) 3.45
(E) 7.00
8. Arrange the acid strength for the followings according to an increasing order. (p.p.
670-674)
(a) CI3COOH, CBr3COOH, CCl3COOH, CF3COOH, CH3COOH (b) CI3COOH,
CI2HCOOH, CIH2COOH, CH3COOH, CICl2COOH (c) H2SeO3, H2SeO4, H2SO4, HClO4
(a) CH3COOH < CI3COOH < CBr3COOH < CCl3COOH < CF3COOH
(b) CH3COOH < CIH2COOH < CI2HCOOH < CI3COOH < CICl2COOH
(c) H2SeO3 < H2SeO4 < H2SO4 < HClO4
Oxoacids having different central atoms (Z) that are from the same group
and that have the same oxidation number. Acid strength increases with
increasing electronegativity of Z. Oxoacids having the same central atom (Z)
but different numbers of attached groups.Acid strength increases as the
oxidation number of Z increases. Strength of carboxylic acid depends on the
nature of group R. The presence of electronegative atom in R draw the
electrons toward R, which makes the acids more stronger.
10. Predict the pH(>7,<7,=7) of the following solution:
NaBr, K2SO3,NH4NO2,Cr(NO3)3
11. Which of the following 0.1 M aqueous solutions is more soluble in a base?
(Application of Example 15.4, p. 679)
(A) NaCl
(B) CaCl2
(C) BaSO4
(D) Na2S
(E) NH4Cl
12. Which of the following is a Lewis acid? (See Example 15.5, p. 683)
(A) NH3
(B) CO32(C) Al3+
(D) BF4-
(E) CN-
A Lewis acid is a substance that can accept a pair of electrons. Lewis acid usually
are cations or electron deficient species.
A Lewis base is a substance that can donate a pair of electrons. Lewis bases
are usually anions or species with lone pairs.