EVEN MORE EQUILIBRIUM PRACTICE!!!!
FREE RESPONSE CALCULATION
NH3 (aq) + H2O (l)  NH4+ (aq) + OH- (aq)
1. In aqueous solution, ammonia reacts as represented above. In 0.0180 M NH 3 (aq) at 250C, the hydroxide ion
concentration, [OH-] is 5.60 x 10-4. In answering the following, assume that temperature is constant at 25 oC
and that volumes are additive.
(a) Write the equilibrium constant expression for the reaction represent above.
(b) Determine the pH of 0.0180 M NH3 (aq).
(c) Determine the value of the base ionization constant, Kb for NH3 (aq).
(d) Determine the percent ionization of NH3 in 0.0180 M NH3 (aq).
(e) In an experiment, a 20.0 mL sample of 0.0180 M NH 3 (aq) was placed in a flask and titrated to the equivalence point and
beyond using 0.0120 M HCl (aq)
(i)
Determine the volume of 0.0120 M HCl (aq) that was added to reach the equivalence point.
(ii)
Determine the pH of the solution in the flask after a total of 15.0 mL of 0.0120 M HCl (aq) was added.
(iii) Determine the pH of the solution in the flask after a total of 40.0 mL of 0.0120 M HCl (aq) was added.
MULTIPLE CHOICE
Use these answers for questions #1-3
(a) Arrhenius acid
(b) Bronsted acid
1.
2.
3.
(c) Bronsted base
(d) Lewis acid
BF3 in the reaction BF3 + F-  BF4
CN- in the reaction: Cu2+ (aq) + 4CN- (aq)  Cu(CN)42- (aq)
H2O in the reaction: HC2H3O2 (aq) + H2O  C2H3O2- (aq) + H3O+ (aq)
4. Which choice shows the correct arrangement of the acids in order of increasing acid strength?
(a) HClO4, HClO3, HClO2, HClO
(b) HClO4, HClO, HClO3, HClO2
(c) HClO, HClO2, HClO3, HClO4
(d) HClO3, HClO, HClO4, HClO2
(e) More information is needed to answer the question.
(e) Lewis base
5. Which of the following is a conjugate acid-base pair?
(a) H3O+ /OH(b) HCl /OCl(c) NH4 /NH3+
(d) H2SO4 /SO42- (e) HClO4/ClO4-
6. The pH of a 0.100 M solution of an aqueous weak acid (HA) is 4.00. The K a for the weak acid is
(a) 1.0 x 10-4
(b) 4.0 x 10-4
(c) 1.0 x 10-7
(d) 1.0 x 10-8
(e) 4.0 x 10-8
FREE RESPONSE ESSAY QUESTION
A 30.00 milliliter sample of a weak monoprotic acid was titrated with a standardized solution of NaOH. A pH meter was used to
measure the pH after each increment of NaOH was added, and the curve above was constructed.
(a) Explain how this curve could be used to determine the molarity of the acid.
(b)Explain how this curve could be used to determine the dissociation constant K A of the weak monoprotic acid.
(c) If you were to repeat the titration using a indicator in the acid to signal the endpoint, which of the following indicators should
you select? Give the reason for your choice.
Methyl red
Ka = 1x10-5
Cresol red
Ka = 1x10-8
Alizarin yellow Ka = 1x10-11
(d) Sketch the titration curve (use the above graph) that would result if the weak monoprotic acid were replaced by a strong
monoprotic acid, such as HCl of the same molarity. Identify differences between this titration curve and the curve shown above.