Solubility Rules!
A basic knowledge of which compounds are
soluble in aqueous solutions is essential
for predicting whether a given reaction
might involve formation of a precipitate (an
insoluble compound).
The following guidelines are
generalizations. A substance is classified
as insoluble if it precipitates when equal
volumes of 0.1 M solutions of its components
are mixed. Keep in mind, however, that no
substance is completely insoluble.
Substances listed as insoluble are, at some
level, partially soluble. The magnitude of
the ion product constant (Ksp)
for the appropriate solubility equilibrium
should be examined. Larger Ksp
values indicate greater solubility; smaller
Ksp values
indicate lesser solubility.
The symbol "<=>" is used here to signify
the 'double-arrrow' symbol for a chemical
equilibrium. The symbol "=>" is used here to
signify the 100% dissociation of a compound
into its electrolyte ions in aqueous
solution. The subscript "(s)" following a
species indicates that it is a solid. The
subscript "(aq.)" following a species
indicates that it is in aqueous solution.
Rule 1. All compounds of Group IA
elements (the alkali metals) are soluble.
For example, NaNO3, KCl, and
LiOH are all soluble compounds. This means
that an aqueous solution of KCl really
contains the predominant species K+
and Cl- and, because KCl is
soluble, no KCl is present as a solid
compound in aqueous solution:
KCl(s) => K+(aq.)
+ Cl-(aq.)
Rule 2. All ammonium salts (salts
of NH4+) are
soluble.
For example, NH4OH is a
soluble compound. Molecules of NH4OH
completely dissociate to give ions of NH4+
and OH- in aqueous solution.
Rule 3. All nitrate (NO3-),
chlorate (ClO3-),
perchlorate (ClO4-),
and acetate (CH3COO-
or C2H3O2-,
sometimes abbreviated as Oac-)
salts are soluble.
For example, KNO3 would be
classified as completely soluble by rules 1
and 3. Thus, KNO3 could be
expected to dissociate completely in aqueous
solution into K+ and NO3-
ions: KNO3 => K+(aq.)
+ NO3-(aq.)
Rule 4. All chloride (Cl-),
bromide (Br-), and iodide (I-)
salts are soluble except for those of
Ag+, Pb2+, and Hg22+.
For example, AgCl is a classic insoluble
chloride salt:
AgCl(s) <=> Ag+(aq.)
+ Cl-(aq.) (Ksp
= 1.8 x 10-10).
Rule 5. All sulfate ( SO4=)
compounds are soluble except those of
Ba2+, Sr2+, Ca2+,
Pb2+, Hg22+,
and Hg2+, Ca2+ and Ag+
sulfates are only moderately soluble.
For example, BaSO4 is
insoluble (only soluble to a very small
extent):
BaSO4(s) <=> Ba2+(aq.)
+ SO42-(aq.)
(Ksp = 1.1 x 10-10).
Na2SO4 is completely
soluble:
Na2SO4(s) => 2 Na+(aq.)
+ SO42-(aq.).
Rule 6. All hydroxide (OH-)
compounds are insoluble except those
of Group I-A (alkali metals) and Ba2+,
Ca2+, and Sr2+.
For example, Mg(OH)2 is
insoluble (Ksp = 7.1 x 10-12).
NaOH and Ba(OH)2 are soluble,
completely dissociating in aqueous solution:
NaOH(s) => Na+(aq.)
+ OH-(aq.), a strong
base
Ba(OH)2(s) => Ba2+(aq.)
+ 2OH-(aq.) (Ksp
= 3 x 10-4)
Rule 7. All sulfide (S2-)
compounds are insoluble except those
of Groups I-A and II-A (alkali metals and
alkali earths).
For example, Na2S(s)
<=> 2Na+(aq.) + S2-(aq.)
MnS is insoluble (Ksp = 3
x 10-11).
Rule 8. All sulfites (SO3=),
carbonates (CO3=),
chromates (CrO4=), and
phosphates (PO43-) are
insoluble except for those of NH4+
and Group I-A (alkali metals)(see rules 1
and 2).
For example, calcite, CaCO3(s)
<=> Ca2+(aq.)
+ CO3=(aq.)
(Ksp = 4.5 x 10-9). |
