Chem 1721/1821
Name _______________________________
Quiz 6 (25 points)
April 11, 2013
pX = −log X
pKw = pH + pOH
!=
1.
!!± ! ! !!"#
Kw = 1 x 10—14 at 25ºC
pKw = pKa + pKb
!" = !! ! + !"#
!"
[!"#$]
[!"#$]
Given below are 8 different chemical equations (A – H). Indicate the one best equation that represents the equilibrium or
reaction described in questions a – c.
A. HCO3— (aq) + H2O (l)  CO32— (aq) + H3O+ (aq)
B. HCO3— (aq) + H2O (l)  H2CO3 (aq) + OH— (aq)
C. CO32— (aq) + H2O (l)  HCO3— (aq) + OH— (aq)
D. HCO3— (aq) + H+ (aq)  H2CO3 (aq)
E. HCO3— (aq) + OH— (aq)  CO32— (aq) + H2O (l)
F. CO32— (aq) + OH— (aq)  HCO3— (aq) + O2— (aq)
G. CO32— (aq) + H+ (aq)  HCO3— (aq)
H. HCO3— (aq) + CO32— (aq)  H+ (aq) + 2 CO32— (aq)
a.
Which equation describes the equilibrium that controls the pH in a solution of K2CO3 (aq)?
b.
Which equation describes the equilibrium that controls the pH in a buffer solution composed of NaHCO3 and
Na2CO3?
c.
________
________
Which equation describes the reaction that occurs when a small amount of strong acid is added to a buffer solution
composed of NaHCO3 and Na2CO3?
________
2.
Using the Henderson-Hasselbalch equation, calculate the pKa of butyric acid (HC4H7O2) if a buffer solution composed of
2.22 M butyric acid and 2.55 M potassium butyrate (KC4H7O2) has pH = 4.88.
3.
Shown below is a titration curve from a weak acid + strong base titration. Use this graph to answer questions a – c.
a.
What is the (approximate) volume of base that is
required to reach the stoichiometric point in this
titration?
_________________
b.
What is the (approximate) pH of the solution at the
stoichiometric point in this titration?
_________________
c.
Determine the (approximate) pKa of the acid.
_________________
4.
Calculate the pH of a 1.025 M solution of sodium sulfite, Na2SO3. For HSO3—, Ka = 6.2 x 10—8.
pH = _______________
5.
Circle ALL of the salts below that should result in aqueous solutions with pH < 7 at 25ºC.
KHCO3
6.
7.
Sr(ClO4)2
C2H5NH3NO3
Al(H2O)63+
c. when excess H+ is added
a. always
b. never
d. when [acid] = [base]
e. when buffer pH changes by > 1 pH unit
Pyridine (C5H5N) is a weak base with Kb = 1.4 x 10—9.
a.
Determine the Ka of the conjugate acid of pyridine.
b.
Indicate whether each of the following quantities will increase, decrease, or remain constant when some pyridinium
bromide (C5H5NHBr) is added to a solution of pyridine.
_____________________
% ionization _____________________
Consider a buffer solution composed of triethanolamine (C6H15O3N) – a weak base with Kb = 5.9 x 10—7, and
triethanolamonium chloride (C6H15O3NHCl). Indicate whether each of the following quantities will increase, decrease,
or remain constant when a small amount of OH— is added to the buffer solution.
[base]/[acid] _____________________
9.
KF
When is the pH of a buffer solution equal to the pKa of its acid? Circle the one best answer below.
pOH ____________________
8.
NaCN
a.
pH _____________________
50.0 mL of 0.635 M HNO3 (aq) is titrated with 0.220 M Ca(OH)2 (aq). Determine the pH of solution after the
addition of 85.0 mL of base.
pH = _______________
Is this before, at, or beyond the stoichiometric point of this titration?
b.
____________________
20.0 mL of 0.774 M quinoline (C9H7N, Kb = 6.3 x 10—10) is titrated with 0.500 M HBr (aq). Determine the pH of the
solution after the addition of 10.0 mL of acid.
pH = _______________
c.
Determine the pH at the stoichiometric point in the titration of 100.0 mL of 0.805 M cyanic acid (HOCN,
Ka = 3.5 x 10—4) with 1.05 M NaOH (aq).
Determine the volume of NaOH required to reach stoichiometric point.
________________ mL
pH = _______________