Ion Product Quotient: Predicting Precipitation

The higher the Ksp value, the more
soluble the compound

Solubility
◦ Of a substance is the maximum concentration a substance
can have in water to make a saturated solution.

Once the concentration exceeds the solubility, it will
accumulate at the bottom of the mixture as a
precipitate (supersaturation).

Every time we look at a solution in equilibrium, we
are dealing with a saturated solution.

Most substances, even ‘insoluble’ substance will
dissolve to some degree.

When two solutions are mixed together the
system will try to reach equilibrium.

Ksp is the equilibrium expression used when
a solution is saturated

When the solution is not saturated, Qsp is
usually used to describe the equilibrium
◦ You can think of Qsp as a ‘trail Ksp’
◦ Q has the same form as K, but typically involves
non-equilibrium concentrations

Once the Qsp is calculated, we compare it to
the actual Ksp for the substance.
◦ Precipitation can be predicted by comparing Qsp
and Ksp

We will get one of 3 situations:

A precipitate will form.
◦ This means that the solution looks at the trial
situation and says that there are too many ions for
that specific temperature. It will kick the ions out as
precipitates.

Saturated salt will precipitate until Qsp = Ksp

A saturated solution has formed. There is the
perfect amount of ions present. No more salt
can dissolve.

Solution is at equilibrium

No precipitate forms.



An unsaturated solution has formed.
More salt can dissolve without a precipitate
forming
This means that there is no precipitate that
forms and there is room for more ions.


One other necessary discussion is that if I mix
two substances together, they dilute each
other.
This dilution must be considered when
deciding on whether a precipitate will form.

1. Decide if any possible precipitates can form from the
two substances being mixed using a solubility table.
◦ Write the net ionic equation for this substance to determine
what the trial Qsp expression is.

2. Calculate the dilution of both ions that will form the
precipitate.
◦ Note that the final volume will be the sum of the volumes of
both substances.

3. Using the concentrations above, calculate the trial
Qsp

4. Compare to the actual Ksp to determine whether a
precipitate will form.

25.0 mL of 0.00200 M of potassium
chromate are mixed with 75.0 mL of
0.000125 M of lead (II) nitrate. Will a
precipitate form if Ksp of lead (II) chromate is
1.8 x 10-14?