10. What is the pH of a solution of 0.200M
H
3
C
6
H
5
O
7
(citric acid)?
K
a
=
3.5 x 10
-4
Ionization equilibrium:
H
3
C
6
H
5
O
7
(aq)
H
+
(aq) + H
2
C
6
H
5
O
7
-
(aq)
I.C.E. table:
H
3
C
6
H
5
O
7
(aq)
H
+
(aq)
H
2
C
6
H
5
O
7
-
(aq)
INTIAL
0.200M
0M
0M
CHANGE
-xM
+xM
+xM
EQUILIBRIUM
(0.200-x)M
xM
xM
K
a
=3.5 x
10
-4
= {x
2
} / {(0.200-x)}
We can solve for
x
to get 8.4 x
10
-3
M =
[ H
+
]
.
What is the final step?
a) find the pH by pH = - log
(K
a
)
b) find the
K
a
by substituting this
[ H
+
]
into the
K
a
expression
c) find the pH by pH = - log
[ H
+
]
