The Solubility Product Constant, Ksp
Key Vocabulary
Saturated Solution – a solution containing the maximum quantity of a solute at a specific
temperature and pressure.
Supersaturated Solution –
Strong Electrolyte – salt solutions that contain highly soluble salt e.g. NaCl(aq) or MgSO4(aq)
Weak Electrolyte –
Solubility –
Solubility Equilibrium –
Solubility Product Constant (Ksp) –
Heterogeneous Equilibrium: A Solubility System








X-ray technicians give patients barium sulphate
suspension prior to x-ray
barium sulphate is opaque to x-rays, helping to
define body
barium sulphate crystals added to water, barium
and sulphate ions leave the surface of solid and
enter the solution (forward change, dissolving
occurs at a greater rate than the reverse change
BaSO4(s)
Ba2+(aq) + SO42-(aq)
As more ions enter the solution, the rate of the
reverse change, recrystallisation, increases.
Eventually, the rate of recrystallisation becomes
equal to the rate of dissolving
System is than considered at equilibrium
Since reactants and products are in different
phases, the reaction is said to have reached
heterogeneous equilibrium
1
The Solubility Product Constant

when excess solid is present in a saturated solution, we can write the equilibrium
constant expression for the dissolution of the solid in the same way that we wrote the
equilibrium constant expression for a homogenous equilibrium
[ Ba 2 ( aq) ][ SO4
Kc 
BaSO4( s )

2
( aq )
]
the concentration of a solid, however, is itself a constant at a constant temperature,
therefore one can combine the term for the concentration of the solid with the
equilibrium constant to arrive at a new constant
2
2
(
aq
)
c
4( s )
4 ( aq )
K [ BaSO
]  [ Ba
][ SO
K sp  [ Ba 2  ( aq) ][ SO4
2
( aq )
]
]

the new constant is called the solubility product constant, Ksp

In general, for the dissociation equilibrium equation BC(s)
bB+(aq) + cC-(aq)
where BC(s) is a slightly soluble salt, and B+(aq) and C-(aq) are aqueous ions
Ksp = [B+(aq)]b[C-(aq)]c
The Ksp equals the ION PRODUCT of a saturated solution in which dissolved and undissolved
solutes are in DYNAMIC EQUILIBRIUM.
For example, Li2CO3(s)
Ksp values can be found on p. 802 Table C8
Example 1
Write the balanced chemical equation that represents the dissociation of each compound in
water. Then write the corresponding solubility product expression.
(a) calcium phosphate
(b) barium fluoride
2
Calculating Ksp from Solubility Values
Example 2
A chemist finds that the solubility of silver carbonate, Ag2CO3, is 1.3 x 10-4 mol/L at 25ºC.
Calculate Ksp for the silver carbonate.
Calculating Solubilities from Ksp values
Example 3
Calculate the solubility of iron(II) carbonate at 25°C. The Ksp of FeCO3(s) is 3.5 x 10-11 at 25°C
3