Experiment C6- Cations - 1 EXPERIMENT C6 QUALITATIVE...

1
E
XPERIMENT
C6: Q
UALITATIVE
A
NALYSIS OF
C
ATIONS
Learning Outcomes
Upon completion of this lab, the student will be able to:
1) Analyze a given sample of an ionic compound and identify which of the
following cations are present: silver, lead (II), bismuth (III), iron (III),
manganese (II), aluminum, chromium (III), barium, strontium, calcium,
nickel (II), copper (II), magnesium, zinc
Introduction
The cations being tested in the qualitative analysis scheme are organized into
groups labeled A through D (Table 1)
Group Cations
A Ag
+
, Pb
2+
B Bi
3+
, Fe
3+
, Mn
2+
, Al
3+
, Cr
3+
C Ba
2+
, Sr
2+
, Ca
2+
D Ni
2+
, Cu
2+
, Mg
2+
, Zn
2+
T
ABLE
1
This grouping of ions was arrived at based on the differences in the solubility of
these ions.
G
ROUP
A: Ag
+
, Pb
2+
The K
SP
of the chloride salts of the two ions in this group is given in Table 2.
Salt K
SP
AgCl 1.7 × 10
-10
PbCl
2
1.6 × 10
-5
T
ABLE
2
The chloride salts of all the other cations that will be analyzed in this experiment are
highly soluble. Therefore, addition of aqueous HCl will result in a precipitate
containing chlorides of the Group A cations.
Ag
+
(aq)
+ Cl
−
(aq)
! AgCl
(s)
Pb
2+
(aq)
+ 2Cl
−
(aq)
! PbCl
2(s)

2
The chlorides of all other cations will remain in solution. When this mixture is
centrifuged: the precipitate will contain the group A cations and the supernatant
will contain the cations from the other groups.
Of the two chloride precipitates, PbCl
2
is soluble in hot water and AgCl is soluble in
aqueous ammonia.
Test for Pb
2+
When the chloride precipitates of the Group A cations are mixed with hot water,
PbCl
2
will dissolve. This is separated from AgCl by centrifuging the mixture. The
PbCl
2
in the supernatant can be confirmed by reacting the supernatant with a
solution of KI, which results in a yellow precipitate of PbI
2
.
PbCl
2(s)
+ H
2
O
(l)
+ heat ! Pb
2+
(aq)
+ 2Cl
−
(aq)
Pb
2+
(aq)
+ 2KI
(aq)
! PbI
2(s)
+ 2K
+
(aq)
Test for Ag
+
The AgCl precipitate dissolves in aqueous ammonia due to the formation of a
complex ion.
AgCl
(s)
+ 2NH
3(aq)
! Ag(NH
3
)
2(aq)
+ Cl
−
(aq)
Addition of HNO
3
results in the decomposition of the silver ammonium complex.
The free silver ions combine with the chloride ions from the above reaction to form
the AgCl precipitate again.
Groups A, B, C, D
Add 6M HCl
Precipitate: Group A-
AgCl, PbCl
2
Supernatant: Groups
B, C, D

3
G
ROUP
B: (Bi
3+
, Fe
3+
, Mn
2+
, Al
3+
, Cr
3+
)
The hydroxides of the cations in this group are almost completely insoluble. The K
SP
of the hydroxide salts are given in Table 3.
Salt K
SP
Mn(OH)
2
4.5 × 10
-14
Fe(OH)
3
1.0 × 10
-36
Bi(OH)
3
Insoluble
Al(OH)
3
3.7 × 10
-15
Cr(OH)
3
6.7 × 10
-31
T
ABLE
3
The hydroxide salts of the Group C cations are far more soluble. Although, the
hydroxide salts of the Group D cations are also quite insoluble, the distinction
between the hydroxide salts of Group B cations vs. Group D cations is that, the
hydroxide salts of the Group D cations are soluble in aqueous ammonia, whereas
those of the Group B cations are not.