3. Calculate the concentration of free
Cr
3+
ion when 0.010 mol of
Cr(NO
3
)
3
is dissolved in a litre of solution buffered at pH = 10.0, and the
K
f
for
Cr(OH)
4
-
= 8.0 x 10
29
.
Equilibrium equation:
Cr
3+
(aq) + 4 OH
-
(aq)
Cr(OH)
4
-
(aq)
How do we calculate the equilibrium concentration of
OH
-
in solution?
a)
[ OH
-
]
= + log (pH) = 1.0 M
b)
[ OH
-
]
= + log (pOH) = 1.0 M
c)
[ OH
-
] = {1.0 x 10
-14
} / {10
-pH
} = 1.0 x 10
-4
M
d)
[ OH
-
] = {1.0 x 10
-14
} / {- log [ H
+
]} = 1.0 x 10
-15
M
