CHEM 112-2011
NAME: _______________________
Vining, Exam #3
SHORT ANSWER QUESTIONS ============= 4 points each ============ All work must be shown.
+
1. What are [H3O
-
] and [OH ] for a solution that has a pH of 9.0?
Choose one.
(a) [H3O+] = 9 M and [OH ] = 5 M
(b) [H3O+] = 5 M and [OH ] = 9 M
(c) [H3O+] = 1 x 10-9 M and [OH ] = 1 x 10-5 M
(d) [H3O+] = 1 x 10-5 M and [OH ] = 1 x 10-9 M
(e) [H3O+] = 1 x 109 M and [OH ] = 1 x 105 M
2. What is the conjugate base of HC2O4 ?
(a) H2C2O4
3.
(b) C2O42-
(d) H3C2O4+
(c) 2 CO2
a. Will a solution of NH4Br be
acidic
basic
neutral
b. Will a solution of NaCN be
acidic
basic
neutral
c. Will a solution of NaNO3 be
acidic
basic
neutral
4. Will Cu3(PO4)2 be more soluble in pure water, or in water in which Na3PO4 has been dissolved?
Cu3(PO4)2(s)  3 Cu2+(aq) + 2 PO43-(aq)
circle answer: pure water or Na3PO4 solution
5. Consider the following two reaction steps. Add the following labels to the species they represent. Point to the species
and add the label. You label four things in total. If something is both Lewis and Bronsted, label it Bronsted.
BA = Bronsted Acid
BB = Bronsted Base
LA = Lewis Acid
LB = Lewis Base
6. Examine the Table of Acids and Bases on the last page.
a. Which acid is stronger:
H2CO3
or
b. Which base is stronger:
HCO2-
or
NH4+
CN-
7. Which acid base pair would be best to use for creating a buffer of pH = 3. Circle one.
-
a. HF/F-
b. CH3CO2H/CH3CO2
c. NH4+/NH3
d. HCO3-/CO32-
8. Consider the alpha plot below, which is for an acid-base system HA/A-.
a. which species is mainly prevalent at pH = 8?
HA
or
A
-
b. What is pKa of HA?
c. What is Ka of HA?
_____________
_______________
9. Compare the Ksp values from the data sheet. Which is more soluble (in mol/L):
PbSO4
or
PbCrO4
10. Which of the following mixtures will form a buffer solution (in 1 L water). Circle all correct choices.
0.10 mol CH3CO2H + 0.070 mol NaCH3CO2
0.10 mol HCl + 0.30 mol NaCH3CO2
0.10 mol NaOH + 0.10 mol NaCH3CO2
0.10 mol NaCl + 0.10 mol CH3CO2H
11. Write the Ksp expression for the dissolution of AlPO4.
12.
Write the Ka expression for H3PO4.
2-
Write the Kb expression for CO3 .
13. Se and Te have very similar electronegativity values. In general, larger atoms form weaker bonds. Which should be
the stronger acid:
H2Se
or
H2Te
14. A buffer is made containing NH3 and NH4Cl. Write net-ionic equations for:
The buffering reaction that occurs when HCl is added to the buffer:
The buffering reaction that occurs when NaOH is added to the buffer:
15. When choosing an acid-base titration indicator, you want:
(circle one)
a) the pH of the indicator to be close to the pH of the titration equivalence point
b) the pKa of the indicator to be close to the pKa of the acid being titrated
c) the pKa of the indicator to be close to the pH of the titration equivalence point
d) the pKb of the base form of the indicator to be close to the Ka of the acid being titrated
LONG ANSWER QUESTIONS ============= 10 points each ============ All work must be shown.
16. a. What is the solubility of CaF2, in grams per liter? Molar mass = 78.08 g/mol.
______________ g /L
b. What is the solubility of CaF2 in a solution that contains 0.50 M NaF? In moles per liter.
_____________ mol/L
17. What is the pH of a 0.80 M solution of CH3CHO2H?
Ka = 1.8 x 10-5
Show your work, the ICE method must be used and presented correctly. Quadratic equation is not needed for this.
pH = _______
18.
Consider the titration curve below involving a weak acid (HA) and a weak base (A-).
a. Which does this titration represent? Circle it.
weak acid being titrated with NaOH
or
weak base being titrated with HCl
b. What is the approximate value of Ka for the acid
form of the studied acid-base pair?
Ka = ___________________
c. What acid-base species is/are mainly in solution at point A? _____________________
d. What acid-base species is/are mainly in solution at point B? _____________________
e. What acid-base species is/are mainly in solution at point C? _____________________
19. a.
What is the pH of a buffer solution made by mixing 100 mL of a 0.14 M H2PO4- and 100 mL of a 0.55 M HPO42-?
Ka(H2PO4-) = 6.2 x 10-8
Ka(HPO4-) = 3.6 x 10-13
pH = _____________
b. If 50 mL of water is added to this buffer, the pH will:
a. increase
b. decrease c. remain nearly constant
pH = -log[H3O+]
pOH = -log[OH-]
[H3O+] = 10-pH
pKa = -log(Ka)
pH = pKa + log([base]/[acid])
[OH-] = 10-pOH
Ka and Kb values for Common Weak Acids and Bases