Nov. 20
th
, 24
th
Tutorial Notes
1)
The solubilities of AgI and Bi
2
S
3
in water are 1.2x10
8
mol kg
1
and 1.6x10
20
mol kg
1
respectively, at 25
o
C.
(Note: The solubility for Bi
2
S
3
previously posted on the website
was incorrect.) Calculate their solubility constants.
Is there any significant difference
when activity coefficients are ignored?
(i) The dissociation reaction for silver iodide is:
AgI(s)
→
Ag
+
(aq) + I

(aq)
Therefore if the solubility of this salt is 1.2x10
8
mol kg
1
then the concentration of each
ion is:
+

8
1
Ag
8
I
1
1.2
10
mol kg
1.2
10
1 mol kg
o
o
b
b
b
b
!
!
"
=
=
=
"
.
For monovalent salts, the
concentration equals the ionic strength of the solution – this can be used to calculate the
mean activity coefficient using the limiting law:
(
)
Ag
I
8
5
log
1
1 0.509
1.2
10
5.58
10
1.0
z
z
A
I
!
!
+
"
±
"
"
±
=
"
=
"
#"
#
=
"
#
=
Since
1
!
±
=
then:
(
)
+

+

Ag
I
Ag
I
2
8
16
1.2
10
1.4
10
sp
o
o
K
a
a
b
b
b
b
!
!
=
=
=
"
=
"
Therefore the solubility product for AgI is 1.4x10
16
and there is no significant difference
when the mean activity coefficient is ignored.
(ii) The dissociation reaction for Bi
2
S
3
must produce a neutral solution.
We know that
there are 2 Bi ions and 3 S ions produced in this reaction, although it may not be
immediately apparent what the charge number might be.
However, if the 2 Bi cations are
to be charged balanced by 3 Si ions then 2Bi
v+
= 3S
v
.
The lowest integer values that
satisfy this relationship are
v
+
=3,
v

=2.
From its position on the periodic table we know
This preview has intentionally blurred sections. Sign up to view the full version.
View Full Document
that S
2
will have a completely filled porbital (and is therefore a relatively stable
ionization state).
Therefore the dissociation reaction is:
Bi
2
S
3
(s)
→
2Bi
3+
(aq) + 3 S
2
(aq)
The solubility
s
= 1.6x10
20
mol kg
1
. Therefore the concentration of the ions in this
solution are:
(
)
3+
20
1
20
Bi
1
2
1.6
10
mol kg
2
3.2
10
1 mol kg
o
o
b
s
b
b
!
This is the end of the preview.
Sign up
to
access the rest of the document.
 Fall '08
 GIORGI
 Electrochemistry, Reaction, Solubility, mol kg

Click to edit the document details