Equilibrium Principles in Aqueous Systems

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EQULIBRIUM

Ks is the solubility PRODUCT, or the equilibrium constant and can be calculated using the formula; the greater the value of Ks the more soluble the substance

Ks = [product] x [product]

[reactant] x [reactant]

NOTE: Keep out solids and liquids in the equation

When determining the Ks (solubility product) there can be two scenarios; an AB type salt, or an A2B/AB2

With an AB type salt, the ratio is 1:1 therefore from the equation we can say that the [product] = [product]. Hence; Ks = s^2

While with an A2B/AB2 type salt, the ratio is 2:1 or 1:2 therefore from the equation we know that we'll have to times one of the [product] that is the 2 in the 1:2 ratio by two and square it. Therefore; Ks = 4s^3

In order to find out the solubility when the Ks is already given, the opposite just has to be done.

The Dilution Factor takes into account that when a solution of one kind is added to another, the concentration will decrease proportionally to the volumes and molarity present. If equal volumes of the solution are being added then the volume is simply divided by two. However if unequal volumes of the solution are being added then the following formula is used;

[new] = volume used x [original]

total volume

The Common Ion Effect. If a solution being added contains either one of the ions already present in the solution, then this will REDUCE the solubility of the salt. This is because equilibrium will shift and favour towards the precipitated side of the salt. In order to calculate the solubility of the salt the same formula is used as Ks. However different assumptions will be made;

the concentration of the common ion is [C] + s, since the final concentration of [C] is given by the concentration originally in the solution PLUS the extra dissolved from the salt to make a saturated solution

the s added into [C] is no insignificant and is therefore negligible since the salt is only sparingly soluble in an common ion solution

Qs which is also known as IP (Ionic Product) is much like Ks, and relates to the concentration of products and reactants at any time during a chemical reaction