240 DIY Science: Illustrated Guide to Home Chemistry Experiments
Conversely, if we know the molar solubility, we can calculate the
Ksp. For example, if we know that a saturated solution of silver
bromide at a particular temperature and pressure is 7.2 · 10–7 M,
we can calculate the Ksp of silver bromide as follows:
ksp = (7.2 · 10–7)^2 = 5.2 · 10–13
Until now, we’ve considered only the simplest case, compounds in
which a molecule dissociates into two ions. For ionic compounds
that dissociate into three or more ions, the calculations are a bit
more involved. Consider silver chromate, which dissociates as
follows:
Ag 2 Cro 4 (s) ⇔ 2 Ag+(aq) + Cro 4 2–(aq)
We can no longer use simple squares and square roots, because
each molecule of silver chromate dissociates into three ions—two
silver ions and one chromate ion. We can generalize the solubility
product constants for a salt AxBy that dissociates into the ions
Ay+ and Bx– in the form:
ksp = [Ay+]x · [Bx–]y
so, for silver chromate, Ag 2 (CrO 4 ) 1 , the solubility product
constant can be expressed as:
ksp = [Ag+]^2 ·[Cr 2 o 4 2–]^1
If we assign the molar concentration of the chromate ion as x,
and knowing that the concentration of the silver ion is twice that
of the chromate ion, the expression becomes:
ksp = [2x]^2 · [x]^1 = 4x^3
If we make up a saturated solution of silver chromate, we might
determine by titration that the concentration of silver ion is
1.3 · 10–4 M, from which it follows that the concentration of the
chromate ion must be 0.65 · 10–4 M, which can also be expressed
as 6.5 · 10–5 M. Plugging these values into the equation yields:
ksp = (1.3 · 10–4)^2 · (6.5 · 10–5)^1 = 1.1 · 10–12
If the Ksp is known, we can determine the molar solubility of silver
chromate as follows:
ksp = 1.1 · 10–12 = 4x^3
x^3 = (1.1 · 10–12/4) = 2.8 · 10–13
x = 6.5 · 10–5
SBSTITUTIU oNS ANd modIfICATIoNS- You may substitute uniodized table salt (popcorn or
kosher salt) for the sodium chloride. - You may substitute cream of tartar from the grocery
store for the potassium hydrogen tartrate.
CUTIOA nS
Glacial acetic acid is corrosive, toxic, and has a very
strong, biting odor. Wear splash goggles, gloves, and
protective clothing.z
In principle, you can use solubility product constants to
calculate molar solubilities of ionic salts. In practice, observed
solubilities often differ significantly from such calculated values.
Electrostatic interaction between ions can affect the results,
particularly in any but very dilute solutions, and other equilibria
occurring simultaneously in the solution can have a major
impact. For example, the solubility of magnesium hydroxide
varies greatly with the pH of the solution because of the
common ion effect. Magnesium hydroxide is much more
soluble in acid solutions, which have a very low [OH–] and
correspondingly less soluble in basic solutions, which have
a high [OH–]. Finally, the assumption that ionic substances
fully dissociate in solution into simple solvated ions is not
always true. For example, magnesium fluoride (MgF 2 ) does
not dissociate completely into Mg2+ and F– ions. Instead, some
partially dissociated magnesium fluoride is present as
MgF+ ions.In this lab, we’ll determine solubility product constants for two
compounds, using two different methods.