5 Steps to a 5 AP Chemistry 2019

236 ❯ STEP 4. Review the Knowledge You Need to Score High

1. B—Any acid will have a pH below 7; thus, C
   and D can be eliminated. A 0.1 molar solution
   of a strong acid would have a pH of 1. Acetic
   acid is not a strong acid, which eliminates A.


2. A—The K nearest 10-^8 will give a pH near 8.
   The answer must involve the H 2 PO 4 - ion.


3. D—The two substances are not a conjugate
   acid–base pair, so this is not a buffer. Both com-
   pounds are salts of a strong base and a weak acid;
   such salts are basic (pH > 7).


4. C—The two substances constitute a conjugate
   acid–base pair, so this is a buffer. The pH should
   be near –log Ka1. This is about 2 (acid).


5. B—Any time an acid is added, the pH will
   drop. The reaction of the weak base with the
   acid produces the conjugate acid of the weak
   base. The combination of the weak base and its
   conjugate is a buffer, so the pH will not change
   very much until all the base is consumed. After
   all the base has reacted, the pH will drop much
   more rapidly. The equivalence point of a weak
   base–strong acid titration is always below 7 (only
   strong base–strong acid titrations will give a pH
   of 7 at the equivalence point). The value of pOH
   is equal to pKb halfway to the equivalence point.


6. A—If pH = 4.0, then [H+] = 1 × 10 -^4 = [A-],
   and [HA] = 0.30 – 1 × 10 -^4 ≈ 0.30. The generic
   Ka is

[H+−] [A]

[HA]

, and when the values are entered

into this equation,

(1  10 )× −

(0.30)

42

= 3.3 × 10 -^8.

27. For which of the following equilibriums will
    Kc = Kp?
    (A) H 2 (g) + I 2 (g) ^ 2 HI(g)
    (B) CO(g) + Cl 2 (g) ^ COCl 2 (g)
    (C) PCl 5 (g) ^ PCl 3 (g) + Cl 2 (g)
    (D) N 2 (g) + 3 H 2 (g) ^ 2 NH 3 (g)
    28. CaCO 3 (s)  CaO(s) + CO 2 (g)
    Which of the following is the correct Kp expres-
    sion for the above reaction?

(A) Kp =

P P

P

CaOCO

CaCO

2

3

(B) Kp = [CO 2 ]

(C) Kp =

[] 

 

CaO CO

CaCO

2

3

(D) Kp = PCO 2

❯ Answers and Explanations

Since you can estimate the answer, no actual

calculations are necessary.

7. A—This is an acid-dissociation constant; thus,
   the solution must be acidic (pH < 7). The pH
   of a 0.010 M strong acid would be 2.0. This is
   not a strong acid, so the pH must be above 2.


8. A—A is the salt of a strong acid and a weak base;
   therefore, this salt is acidic. B is a salt of a strong
   acid and a strong base; such salts are neutral.
   C and D are salts of a weak acid and a strong
   base; such salts are basic. Since you can estimate
   the answer, no actual calculations are necessary.


9. B—Sodium nitrite is a salt of a weak acid and
   a strong base. Ions from strong bases (Na+ in
   this case) do not undergo hydrolysis and do not
   affect the pH. Ions from weak acids (NO 2 - in
   this case) undergo hydrolysis to produce basic
   solutions.


10. D—The weak acid and the weak base partially
    cancel each other to give a nearly neutral solution.


11. A—The equilibrium constant expression is Kb =

4.0 × 10 -^10 =

[OH]−+ [CHNH ]

[CHNH]

65 3

65 2

. This expres-

sion becomes

−

()()

(1.0 )

xx

x

= 4.0 × 10 -^10 , which

simplifies to (1.0)

x^2

= 4.0 × 10 -^10. Taking the

square root of each side gives x = 2.0 × 10 -^5 =

[OH-^ ]. Since you can estimate the answer, no

actual calculations are necessary.