CHEM1612
2014-N-13
November 2014
2+
What is [Cd
] when E
reaches 0.015 V?
cell
!
!
(.
)( )
+
[
]
E
= (0.04 V) –
= +0.015 V
cell
+
!
( ×
)
[
]
so:
!
[
]
= 7.0
!
[
]
2+
If the change from the initial concentrations is x, [Cd
(aq)] = (0.200 – x) M and
2+
[Fe
(aq)] = (0.800 + x) M:
. –
= 7.0
x = -0.075 M
.!
2+
So that [Cd
(aq)] = (0.200 - 0.075) M = 0.125 M.
Answer: 0.125 M
What are the equilibrium concentrations of both ions?
Using E° =
lnK = 0.04 V gives K = 22.5
so:
!
[
]
= 22.5
!
[
]
2+
If the change from the initial concentrations is x, [Cd
(aq)] = (0.200 – x) M and
2+
[Fe
(aq)] = (0.800 + x) M:
. –
= 22.5
x = 0.158 M
.!
2+
2+
So that [Cd
(aq)] = (0.200 – 0.158) M = 0.042 M and [Fe
(aq)] = (0.800 + 0.158)
M = 0.958 M
2+
2+
[Cd
] = 0.042 M
[Fe
] = 0.958 M