1. A conjugate acid of a weak base
16.75 by forum request
(B) (BH+)
C2H5NH2(aq) + H20   C2H5NH3+ (aq) + OH- (aq)
A conjugate acid of a weak base
A weak base
B 
 BH+
OH-
0.075M -X X
(0.075-X)
Kb = [C2H5NH3+ ] [OH- ] = (X) (X) = X = 6.4 x 10-4
[C2H5NH2] (0.075-X) (0.075-X)

2. Kb = [C2H5NH3+ ] [OH- ] = (X) (X) = X2 = 6.4 x 10-4
[C2H5NH2] (0.075-X) (0.075-X)
X2 = x 10-4
(0.075-X)
X x 10-4 X – 4.8 x 10-5 = 0
X= -6.4x (6.4x10-4)2 - 4(1) (-4.8 x 10-5) :X= 6.61 x 10-5 = [OH-] 2
6.61 x 10-5 = [OH-]
pOH- = 2.18, pH = 11.82

3. NaCN  Na+ + CN- CN- + H2O  OH- + HCN RECOGNIZE: 16.83 a RECOGNIZE:
THE CONJUGATE OF A WEAK ACID, (HAS A Ka) AND WILL HYDROLYZE TO RAISE Ph RELEASING OH- FROM HYDROLYSIS.
16.83 a
NaCN  Na+ + CN-
RECOGNIZE:
THIS IS THE CONJUGATE SALT OF A WEAK ACID
RECOGNIZE:
THE CONJUGATE OF A STRONG BASE (GROUP ONE METAL, NaOH) DOES NOT HYDROLYZE, NO Ph EFFECT
CN- + H2O  OH- + HCN
HYDROLYSIS OF CN-, CONJUGATE BASE.
WEAK ACID, IGNORE ANY IONIZATION AS IT IS INSIGNIFICENT.
CONJUGATE BASE OF HCN

4. HYDROLYSIS OF CN-, CONJUGATE BASE. FOR SAKE OF TIME APPROXIMATE X
CN- + H2O  OH- + HCN
HYDROLYSIS OF CN-, CONJUGATE BASE.
16.83 a
CN- 
 OH-
HCN
0.10 -X X
0.10-X
Look up the Ka of HCN
Kb = [HCN][OH-] WHERE Kb = Kw/Ka = 1 x 10-14/4.9x10-10= 2.0 x10-5
[CN-]
FOR SAKE OF TIME APPROXIMATE X
Kb = [HCN][OH-] = 2.0 x 10-5 = (X)(X)
[CN-] (0.10-X)
2.0 x 10-5 = X2
(0.10)
X2 = (0.10) 2.0x10-5
X = [OH-] =.00143
pOH = 2.85; pH = 11.15

5. Na2CO3(aq) 2 Na+ (aq) + CO32- (aq)
16.83 b: by forum request
REACTION #1 : DISSOCIATION OF CONJUGATE SALT.
Na2CO3(aq) 2 Na+ (aq) + CO32- (aq)
RECOGNISE THIS AS THE SALT OF A WEAK ACID AND STRONG (GROUP ONE METAL) BASE: NaOH .
REACTION #2 : HYDROLYSIS #1 OF CO32- , CONJUGATE BASE.
CO32- + H2O  OH- + HCO3-
THIS IS THE FIRST HYDROLYSIS OF THIS DIBASIC ION. Kb1 = Kw / Ka2
Ka2<< Ka1 therefore
Kb1>> Ka2, use Kb1 for pH
REACTION #3 : HYDROLYSIS #2 OF CO32- , CONJUGATE BASE.
HCO3- + H2O  OH- + H2CO3
THIS IS THE SECOND HYDROLYSIS OF THIS DIBASIC ION. Kb2 = Kw / Ka1

6. 16.83 b
AS USUAL, ASSESS THE STRONGEST IONIZATION FOR THE pH!
CO32-
OH-
HCO3-
0.080 M -X X
0.080-X
Kb = [HCO3-][OH-] WHERE Kb1 = Kw/Ka2 = 1 x 10-14/5.6 *10-11= 1.79 *10-4
[CO32-]
Kb = [HCO3-][OH-] = * = X2
[CO32-] X
1.79 * = X2
0.080-X
0.080 (1.79 * 10-4 ) = X2
X = = [OH-] :pOH- = 2.42 : pH =

7. H3PO4(aq) H+ (aq) + H2PO4- (aq) H3PO4 H+ H2PO4-
MONSTER!
REACTION #1 : DISSOCIATION Ka1 OF POLY (TRI) PROTIC ACID. ALL INOZATIONS ARE WEAK.
H3PO4(aq) H+ (aq) + H2PO4- (aq)
RECOGNIZE THIS AS A WEAK POLYPROTIC ACID IONIZING IN PURE WATER. .
H3PO4 H+
H2PO4-
0.025 M -X X
0.025-X
Ka1 = [H+][H2PO4-]
[H3PO4]
7.5 X 10-3 = [X][X]
[0.025-X]

8. The H2PO4- WILL DISSOCIATE IN Ka2
MONSTER!
7.5 X = [X][X]
[0.025-X]
Assume all H+ is from Ka1, it will appear in the ICE charts of Ka2, Ka3
7.5 X = (X)2
(0.025-X)
0.025-X (7.5 X 10-3 ) = (X)2
X2 + (7.5 x 10-3X )– (1.875 x 10-4)=0
X= X (7.5 X 10-3 )2 - 4(1) ( x 10-4) : X= = [H+]
2(1) X= =[H2PO4- ]
The H2PO4- WILL DISSOCIATE IN Ka2
H2PO4- H+
HPO42- -X X X X

9. H2PO4-(aq) H+ (aq) + HPO42- (aq) H2PO4- H+ HPO42-
MONSTER!
REACTION #2 : DISSOCIATION Ka2 OF POLY (TRI) PROTIC ACID. ALL INOZATIONS ARE WEAK.
H2PO4-(aq) H+ (aq) + HPO42- (aq)
RECOGNIZE THIS AS A WEAK POLYPROTIC ACID IONIZING IN PURE WATER. .
Ka2 = [H+][HPO42-]
[H2PO4-]
H2PO4- H+
HPO42- -X X X X
Ka2 = [0.010+X][X]
[0.010-X]
IGNORE ANY H+ PRODUCED BY Ka2,
ASSUME ALL H+ IS FROM Ka1
Ka2 = [0.010+X][X]
[0.010-X]

10. HPO42- H+ PO43- 6.2x10-8 0.01045 -X X 16.113 MONSTER!
Ka2 = [0.010+X][X]
[0.010-X]
Ka2 = 6.2 x 10-8 = 0.010X
0.010
X = 6.2 X 10-8 = [HPO42-], THE SPECIES OF INTEREST FROM Ka2 FOR Ka3
HPO42- H+
PO43-
6.2x10-8 -X X
6.2x10-8 -X X

11. HPO42-(aq) H+ (aq) + PO43- (aq) HPO42- H+ PO43-
MONSTER!
REACTION #3 : DISSOCIATION Ka3 OF POLY (TRI) PROTIC ACID. ALL INOZATIONS ARE WEAK.
HPO42-(aq) H+ (aq) + PO43- (aq)
RECOGNIZE THIS AS A WEAK POLYPROTIC ACID IONIZING IN PURE WATER. .
Ka2 = [H+][HPO42-]
[H2PO4-]
HPO42- H+
PO43-
6.2x10-8
0.010 -X X
6.2x10-8 -X X
From Ka2
Ka2 = [0.010+X][X]
[6.2x10-8-X]
Ka3 = 4.2 x = 0.010X
6.2x10-8
X = 2.6 x = [PO43-], THE SPECIES OF INTEREST FROM Ka3