To recall the ionic product Q and how it relates to the solubility product Ksp.
How to calculate the ionic product and use it to predict whether a precipitate will form in solution.
As mentioned in the lesson Solubility product; the value of Ksp for a saturated solution of a given compound is constant. It doesn’t matter how the ion concentrations are made up (whether by one solution or multiple combined); if the solution is saturated then the product of the individual ion concentrations will be equal to Ksp.
Using this fact, we can predict if a precipitate forms in the solution or not before making it by using the Ksp value. This Ksp product value is the product of the ion concentrations required to make a saturated solution (for the equilibrium to be established).
If this product of ion concentration is smaller than Ksp, the solution will not be saturated and the equilibrium will not be established.
The ‘product of ion concentration’ is known simply as the ionic product with label Q. As a product, Q is calculated by multiplying the concentrations of ions together. For aqueous ions Mx+ and Xm- forming precipitate MmXx:
Q = [M+]m [X-]x
Compare this to Ksp which is the ion concentration needed to form a saturated solution. Using these two terms, three situations can be described when two solutions are mixed to create a new ionic species.
If Q is smaller than Ksp, then there are less ions in solution than necessary to form a saturated solution. Without a saturated solution, a precipitate will not form.
If Q is equal to Ksp, then there are just enough ions in solution necessary for a saturated solution to form. In this scenario, a solution that has just reached saturation will form. This means that any further addition of aqueous ions will form a precipitate, as the equilibrium shifts to the left to favor the undissolved state.
If Q is greater to Ksp, then there are more ions in solution than are necessary to form a saturated solution. In this scenario, an equilibrium will be established to form a precipitate with the excess ions; a precipitate will form.
For example: a solution made by combining 100 mL of 0.1 M Ca2+(aq) and 75mL of 0.2 M F-(aq) :
Q = [Ca2+][F-]2 To Form CaF2
These solutions dilute each other. Find the new concentration by dividing original volume by combined volume.
[Ca2+]sol = 0.1 M * 0.175L0.1L = 0.057 M
[F-]sol = 0.2 M * 0.175L0.075L = 0.086 M
Q = [0.057] x [0.086]2 = 4.22*10-4
Ksp (CaF2) = 3.45*10-11 M
Q > Ksp therefore a precipitate will form.
Predicting if a solution will form a precipitate.
Use the solubility product expression to predict a precipitate.1
Predicting a precipitate
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