Exercises
Exercise 1. Order the following acids from weakest to strongest using the pKa values given. Calculate the acid dissociation, Ka,values.
a) HSO3–; pKa = 7.2
b) HC2C2O4; pKa = 1.2
c) C6H5OH; pKa = 10
d) H2O; pKa = 14
H2O; < C6H5OH < HSO3– < HC2C2O4
Ka (HSO3–) = 10-pKa = 10-7.2 = 6.3 x 10-8
Ka (HC2C2O4) = 10-pKa = 10-1.2 = 6.3 x 10-2
Ka (C6H5OH) = 10-pKa = 10-10 = 1.0 x 10-10
Ka (H2O) = 10-pKa = 10-14 = 1.0 x 10-14
Exercise 2. An acid has a pKa of 1.8 x 10-5. What is the equilibrium constant, Keq, for its reaction in water? (Hint: Write a balanced equation).
HA (aq) + H2O (l) ⇄ A– (aq) + H3O+ (aq)
pKa (H2O) = 14.0
pKa (HA) = 4.7
Keq = 14.0 – 4.7 = 9.3
Exercise 3. Are the following acidic or basic?
b) shampoo; pH = 6.3 acidic
c) window cleaner; pH = 7.8 basic
d) cranberry juice; pH = 2.5 acidic
e) saltwater fish tank; pH = 8.3 basic
Exercise 4. Write a chemical equation with CH3CH2O– reacting as a base with HBr. Write another equation with CH3CH2OH reacting as an acid with NH3.
CH3CH2O– + HBr ⇄ Br– + CH3CH2OH
CH3CH2OH + NH3 ⇄ CH3CH2O– + NH4+
Exercise 5. Order the following by increasing acid strength.
b) CH3CH2NH2+
c) H2O
d) CH3CH2COOH
e) CH3CH2OH2+
Exercise 6. What are the conjugate bases for the following? Order the bases by increasing basicity.
b) CH3CH2NH2+ CH3CH2NH
c) H2O OH–
d) CH3CH2COOH
e) CH3CH2OH2+
f) CH3CH2OH2+
Exercise 7. Ethene, CH2CH2, has pKa equal to 44. Write an equation, using equilibrium arrows, to show which side equilibrium is favored for the acid-base reaction with OH–. Write another equation for ethene and –NH2. Which is the better base for removing a proton from ethene? (pKa NH3 = 36, pKa H2O = 15.7)
Exercise 8. What is the equilibrium constant, Keq for the reaction between CH3NH2 and H2O?
CH3NH2 + H2O ⇄ CH3NH3+ + OH–
Back to Organic Acids and Bases, pKa and pH, and Equilibrium