AP Chemistry Unit 8- Homework Problems Ka and Kb Acid & Base ...

AP Chemistry
Unit 8- Homework Problems
K a and K b
Acid & Base Theory
1. What is the definition of an Arrhenius acid?
2. What is the definition of an Arrhenius base?
3. What is the definition of a Bronsted acid?
4. What is the definition of a Bronsted base?
5. What is the conjugate base of:
a. HNO 2
b. H 2 SO 3
+1
c. NH 4
d. HCN
6. What is the conjugate acid of:
a. Cl -1
-2
b. HPO 4
c. OH -1
d. N 2 H 4
7. In each equation below, identify each species as the acid, base, conjugate acid or conjugate
base.
a. HNO 3 + PO -3 4 NO -1 3
-2
+ HPO 4
b. N -1 3 + H 2 SO 4 HSO -1 4 + HN 3
c. NH 3 + H 2 PO -1 3 NH +1 4
-2
+ HPO 3
d. H 2 C 2 O 4 + OH -1 -1
H 2 O + HC 2 O 4
8. For each species below, describe whether a solution of it would be acidic, basic, or neutral.
a. HCl h. CaCl 2
b. KOH i. Na 2 CO 3
c. NaCl j. LiBr
d. NaC 2 H 3 O 2 k. KF
e. NH 4 Cl l. NaNO 2
f. Na 3 PO 4 m. K 2 SO 3
g. KNO 3 n. KI
9. What is an amphiprotic substance? List 4 of them.

Auto-Ionization of water, pH, and strong acids
1. What is the equation for the auto-ionization of water?
2. What is the equation for K w and what is its value?
3. Because the pH scale is a logarithmic scale, every change in 1 is really a change in how
much?
4. Using the pH square and the equations learned in class, fill in the blank spots in the chart
below:
pH [H 3 O +1 ] [OH -1 ] pOH
3.5
9.2x10 -9 4.7x10 -11 8.2
5. According to the leveling effect:
a. What is the strongest acid species in water?
b. What is the strongest base species in water?
6. What is the % dissociation of strong acids and bases in water?
7. List 6 strong acids.
8. List 6 strong bases.
Weak acids & bases, K a , K b , and pH
1. How are weak acids and bases different from strong ones?
2. What is the general equation for K a ?
3. What is the general equation for K b ?
4. For a 0.25 M solution of HC 2 H 3 O 2 (K a = 1.8x10 -5 ) calculate:
a. pH
b. % dissociation
5. For a 0.75 M solution of C 2 H 5 NH 2 (K b = 5.6x10 -4 ) calculate:
a. pH
b. % dissociation
6. For a 0.055 M solution of HClO (K a = 2.9x10 -8 ) calculate:
a. PH
b. % dissociation
7. For a 0.0034 M solution of C 5 H 5 N (K b = 1.7x10 -9 ) calculate:
a. pH
b. % dissociation
8. For a 0.15 M solution of H 2 C 6 H 6 O 6 (K a1 = 8x10 -5 K a2 = 1.6 x10 -12 ) calculate:
a. pH
b. Equilibrium concentrations of:
i) HC 6 H 6 O 6
-1
ii) C 6 H 6 O 6
-2
9. For a 0.025 M solution of diprotic base, B (K b1 = 4x10 -6 , K b2 = 8x10 -10 ) calculate:
a. pH
b. Equilibrium concentrations of:
i) HB +
ii) H 2 B +2

Acid-Base Interactions
Strong-Strong
1. What is the pH when 50 mL of 0.1 M HCl is mixed with 50 mL of 0.1 M NaOH?
2. What is the pH when 250 mL of 0.2 M HNO 3 is mixed with 500 mL of 0.1 M KOH?
3. What is the pH when 100 mL of 0.001 M HBr is mixed with 50 mL of 0.001 M NaOH?
4. What is the pH when 40 mL of 0.01 M HI is mixed with 120 mL of 0.008 M CsOH?
5. What is the pH when 50 mL of 0.0025 M H 2 SO 4 is mixed with 75 mL of 0.0040 M LiOH?
6. What is the pH when 20 mL of 0.00050 M HClO 4 is mixed with 20 mL of 0.00040 M Ba(OH) 2 ?
Strong-Weak
7. What is the pH when 25 mL of 0.25 M HCl is mixed with 25 mL of 0.25 M NH 3 ? (K b = 1.8x10 -5 )?
8. What is the pH when 20 mL of 0.020 M HNO 3 is mixed with 20 mL of 0.030 M C 6 H 5 NH 2 (K b = 4x10 -10 )?
9. What is the pH when 50 mL of 0.040 M NaOH is mixed with 50 mL of 0.040 M HN 3 (K a = 1.9x10 -5 )?
10. What is the pH when 120 mL of 0.0075 M KOH is mixed with 200 mL of 0.0050 M HOCl (K a = 3.5x10 -8 )?
Midpoint
11. What is the pH when 25 mL of 0.050 M HCl is mixed with 50 mL of 0.050 C 5 H 5 N (K b = 1.5x10 -9 )?
12. What is the pH when 40 mL of 0.80 M NaOH is mixed with 80 mL of 0.8 M HNO 2 (K a = 4.5x10 -4 )?
13. What is the pH when 30 mL of 0.006 M HNO 3 is mixed with 30 mL of 0.012 M (CH 3 ) 3 N (K b = 7.4x10 -5 )?
14. What is the pH when 100 mL of 0.0078 M KOH is mixed with 100 mL of 0.0156 M C 6 H 5 CO 2 H (K a = 6.3x10 -5 )
K sp and pH
1. What is the pH of a saturated solution of Mn(OH) 2 K sp = 1.9x10 -13 ?
2. What is the pH of a saturated solution of Ca(OH) 2 K sp = 5.5x10 -5 ?
3. At what pH will a 0.0075 M solution of Mg(OH) 2 begin to form a precipitate (K sp = 5.6x10 -12 )?
4. At what pH will a 0.00042 M solution of Pb(OH) 2 begin to form a precipitate (K sp = 1.4x10 -15 )?

Titration Curves
1. Draw a titration curve of a strong base being titrated by a strong acid. Be sure to label a pH of 7 in the
appropriate spot.
pH
mL acid added
2. Draw a titration curve of a strong acid being titrated by a strong base. Be sure to label a pH of 7 in the
appropriate spot.
pH
mL acid added
3. Draw a titration curve of a weak base being titrated by a strong acid. Be sure to label a pH of 7 in the
appropriate spot.
pH
mL acid added

4. Draw a titration curve of a weak acid being titrated by a strong base. Be sure to label a pH of 7 in the
appropriate spot.
pH
mL acid added
7. What is the significance of the midpoint of a titration?
8. If 40 mL of weak acid HA was titrated as the graph below shows, calculate:
a. Show the pH at the equivalence point
b. Show the pH at the midpoint
c. Calculate the original M of HA
d. Calculate K a of HA
Titration of 40 mL of ??? M weak acid HA
with 0.10 M NaOH

9. If 25 mL of weak base B was titrated as the graph below shows, calculate:
a. Show the pH at the equivalence point
b. Show the pH at the midpoint
c. Calculate the original M of B
d. Calculate K b of B
Titration of 25 mL of ??? M weak base B
with 0.50 M HCl
HCl

Buffers
1. What is the definition of a buffer?
2. What kind of an acid and base are present in a buffer?
3. The K a of carbonic acid is 4.3x10 -7 .
a. What is the pH of a solution that is 0.25 M H 2 CO 3 and 0.25 M NaHCO 3 ?
b. What would the ratio of acid to base be if you wanted to buffer at exactly a pH of 6?
4. The K a of hydrozoic acid is 1.9x10 -5 .
a. What is the pH of a solution that is 0.75 M HN 3 and 0.75 M N 3 -1 ?
b. What would the ratio of acid to base be if you want to buffer exactly at a pH of 5?
5. The K b of aniline is 4.3x10 -10 .
a. What is the pH of a solution that is 0.050 M C 6 H 5 NH 2 and 0.050 M C 6 H 5 NH 3 +1 ?
b. What would the ratio of acid to base be if you want to buffer exactly at a pH of 9?
6. The K b of pyridine is 2x10 -9 .
a. What is the pH of a solution that is 1.25 M C 5 H 5 N and 1.25 M C 5 H 5 NH +1 ?
b. What would the ratio of acid to base be if you want to buffer exactly at a pH of 9?
7. Which of the following buffer systems would you choose if you want to buffer at a pH of 2?
-1
a. HC 3 H 5 O 2 / C 3 H 5 O 2 K a = 1.35x10 -5
-1
b. HNO 2 / NO 2 K a = 7.2x10 -4
c. HIO / IO -1 K a = 2.3x10 -11
-1
d. H 3 PO 4 / H 2 PO 4 K a = 7.6x10 -3
Describe how to make the buffer so it is exactly at a pH of 2.
8. Lactic acid has a K a of 8.3x10 -4 . A buffer is made so that it is 0.50 M in HC 3 H 5 O 3 and 0.50
M in C 3 H 5 O 3 -1 .
a. What is the pH of the buffer?
b. What will the pH of the buffer be if 0.15 M of HCl are added?
9. Arsenous acid has a K a of 6.6x10 -10 . A solution is made so that it is 50 mL of 1.2 M in
HAsO 2 .
a. What is the pH of the solution?
b. How many grams of NaAsO 2 must be added so the solution is a buffer at a pH of 10?
Assume no change in volume.
10. Methylamine has a K b of 4.2x10 -4 . A buffer is made so that it is 0.75 M in CH 3 NH 2 and
0.75 M in CH 3 NH 3 +1 .
a. What is the pH of the buffer?
b. What will the pH of the buffer be if 0.25 M of HNO 3 are added?
11. Quinoline has a K b of 6.3x10 -10 . A solution is made so that it is 250 mL of 1.75 M C 9 H 7 N.
a. What is the pH of the solution?
b. How many grams of C 9 H 7 NHCl must be added so that the solution is a buffer exactly
at a pH of 5? Assume no change in volume.

Lewis Acids & Bases
1. What is the definition of a Lewis Acid?
2. What is the definition of a Lewis Base?
3. What is an amphoteric substance?
4. Give 4 examples of amphoteric substances.
5. Aluminum hydroxide, Al(OH) 3 , is a famous amphoteric substance.
a. Write an equation showing Al(OH) 3 acting as a Bronsted base
b. Write an equation showing Al(OH) 3 acting as a Lewis acid
6. In the following equations, which species is acting as the Lewis acid and the Lewis base?
a. NH 3 + BF 3 BF 3 NH 3
b. SnCl 4 + 2 Cl -1 [SnCl 6 ] -2
c. H + + OH -1 H 2 O
Combination Problems
1.

3. For the reaction:
NH 3 (aq) + H 2 O (l) NH 4 +1 (aq) + OH -1 (aq)
In 0.0180 M NH 3 at 25 o C, the [OH -1 ] is 5.6x10 -4 M.
a) Write the equilibrium-constant expression for the reaction above.
b) Determine the pH of 0.0180 M NH 3
c) Determine the value of K b for NH 3
d) Determine the % ionization of NH 3
e) In an experiment, a 20.0 mL sample of 0.0180 M NH 3 was placed in a flask and
titrated to the equivalence point and beyond using 0.0120 M HCl
i) Determine the volume of 0.0120 M HCl that was added to reach the equivalence pt
ii) Determine the pH of the solution in the flask after a total of 15.0 mL of 0.0120 M
HCl was added
iii) Determine the pH of the solution in the flask after a total of 40.0 mL of 0.0120 M
was added.
4. For the reaction:
C 6 H 5 NH 2 (aq) + H 2 O (l) C 6 H 5 NH 3 +1 (aq) + OH -1 (aq)
a) Write the equilibrium constant expression, K b , for the reaction above.
b) A sample of C 6 H 5 NH 2 is dissolved in water to produce 25.0 mL of a 0.010 M solution.
The pH of the solution is 8.82. Calculate K b .
c) The solution prepared in part (b) is titrated with 0.10 M HCl. Calclulate the pH of the
solution when 5.0 mL of the acid has been added.
d) Calculate the pH at the equivalence point of the titration in (c)
e) The pK a values for several indicators are given below. Which is most suitable for this
titration? Justify.
Erythrosine pK a = 3
Litmus pK a = 7
Thymolphthalein pK a = 10
5. Hypochlorous acid, HOCl, is a weak acid. The K a for HOCl is:
K a = [H 3 O +1 ][OCl -1 ] = 3.2x10 -8
[HOCl]
a) Write a chemical equation showing how HOCl behaves as an acid in water.
b) Calculate the pH of a 0.175 M solution of HOCl
c) Write the net ionic equation for the reaction between HOCl and NaOH
d) In an experiment, 20.00 mL of 0.175 M HOCl is titrated with 6.55 mL of 0.435 M NaOH
i) Calculate the number of moles of NaOH added
ii) Calculate the [H 3 O +1 ] in the flask after the NaOH has been added
ii) Calculate the [OH -1 ] in the flask after the NaOH has been added