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The solubility product constant Ksp is just one more
version of the standard equilibrium
constant expression. Here, it's applied to the equilibrium between a solid
and the corresponding ions in solution
- AB(s) < = > A+(aq) + B-(aq)
- Ksp = [A+][B-]
(Remember
that solids do not enter into an equilibrium constant expression.)
To compute the Ksp given the concentrations of ions, use the same
method that you have used to compute K:
write the balanced equation and expression for Ksp, use stoichiometry
to compute the concentration on each ion, then plug in.
Example 1: What is the solubility constant expression for the
following reaction?
- Ca3(PO4)2(s) < = >
3Ca+2(aq) + 2PO4-3(aq)
Solution 1: Simply write down the equilibrium constant expression,
being careful about the coefficients
- Ksp =
[Ca+2]3[PO4-3]2
Example 2: The concentration of Ca+2 in a saturated
solution of Ca3(PO4)2 is found to be
2.95*10-7 M. What is Ksp for calcium phosphate?
Solution 2: Write the equation and the Ksp expression
- Ca3(PO4)2(s) < = >
3Ca+2(aq) + 2PO4-3(aq)
- Ksp =
[Ca+2]3[PO4-3]2
We're given the concentration of Ca+2: we need to find only
the concentration of phosphate ion. Do this by looking at the stoichiometry of
the problem: for every 3 calcium ions formed, 2 phosphates are formed, so
- 2.95*10-7 M Ca+2 * 2 phosphate/3 calcium =
1.97*10-7M PO4-3
Now you know
everything: just plug in
- Ksp =
[Ca+2]3[PO4-3]2 =
(2.95*10-7)3*(1.97*10-7)2 =
1.0*10-33
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