10. Calculate the pH of a 0.050M solution of the weak base pyridine (
C
5
H
5
N
) if it has
K
b
= 1.7
x
10
-4
.
Balanced equation:
C
5
H
5
N (aq) + H
2
O (l)
HC
5
H
5
N
+
(aq) + OH
-
(aq)
I.C.E. table:
C
5
H
5
N (aq)
HC
5
H
5
N
+
(aq)
OH
-
(aq)
CHANGE
-xM
+xM
+xM
EQUILIBRIUM
(0.050-x)M
xM
xM
K
b
= 1.7 x
10
-4
= {x
2
} / {(0.050-x)}
Solving for x, we find that:
x =
9.2 x 10
-6
M
= [OH
-
]
What is the next step?
a) find the pH = - log
[ H
+
]
b) find the pH = - log
[ OH
-
]
c) find the
[ H
+
] = {K
w
} / {[ OH
-
]}
d) find the
[ H
+
] = 10
-pH
