The Foundations of Chemistry

CN 
H 2 O 34 HCN
OH

initial 0.10 M

change due to rxn y M 
y M 
y M

at equil (0.10y) M y M y M

Kb

[HC

[C

N

N

][O

]

H]

2.5 10 ^5

Substitution into this expression gives



(0.

(

1

y

0

)(



y)

y)

2.5^10 ^5 so y1.6^10 ^3 M

y [OH]1.6 10 ^3 M pOH2.80 and pH11.20

The 0.10 MNaCN solution is even more basic than the 0.10 MNaCH 3 COO solution in

part (a).

% hydrolysis100%100%

 1.6% hydrolysis

You should now work Exercises 83 and 84.

The 0.10 Msolution of NaCN is much more basic than the 0.10 Msolution of

NaCH 3 COO because CNis a much stronger base than CH 3 COO. This is expected

because HCN is a much weaker acid than CH 3 COOH so that the Kbfor CNis much

larger than the Kbfor CH 3 COO.

The percent hydrolysis for 0.10 MCN(1.6%) is about 210 times greater than the

percent hydrolysis for 0.10 MCH 3 COO(0.0075%). In Table 18-9 we compare 0.10 M

solutions of CH 3 COO, CN, and NH 3 (the familiar molecular weak base). We see that

CH 3 COOis a much weaker base than NH 3 , whereas CNis a slightly stronger base

than NH 3.

1.6 10 ^3 M



0.10 M

[CN]hydrolyzed



[CN]initial

Ammonium chloride is an ionic salt
that is soluble in water.

TABLE 18-9 Data for 0.10 M Solutions of NaCH 3 COO, NaCN, and NH 3

0.10 MNaCH 3 COO 0.10 MNaCN 0.10 Maq NH 3

Kafor parent acid 1.8 10 ^5 4.0 10 ^10

Kbfor anion 5.6 10 ^10 2.5 10 ^5 Kbfor NH 3 1.8 10 ^5

[OH] 7.5 10 ^6 M 1.6 10 ^3 M 1.3 10 ^3 M

% hydrolysis 0.0075% 1.6% 1.3% ionized

pH 8.88 11.20 11.11

The pH of 0.10 MNaCH 3 COO is

8.88. The pH of 0.10 MNaCN is

11.20. An inert solid has been

suspended in the liquids to improve

the quality of photographs of pH

meters.

SALTS OF WEAK BASES AND STRONG ACIDS

The second common kind of hydrolysis reaction involves the reaction of the cation of a

weak base with water to form nonionized molecules of the weak base and H 3 O
ions.

This upsets the H 3 O
/OHbalance in water, giving an excess of H 3 O
, and making such

solutions acidic. Consider a solution of ammonium chloride, NH 4 Cl, the salt of aqueous

NH 3 and HCl.

18-9