Ksp is really just an equilibrium constant applied to a slightly soluble ionic solid dissolving in water, which means everything already learned about equilibrium expressions carries over directly.
Writing the Ksp expression
For a dissolving solid like AgCl ⇌ Ag+ + Cl−, the Ksp expression is the product of the ion concentrations, each raised to its coefficient: Ksp = [Ag+][Cl−]. The solid itself is never included, exactly like any other equilibrium expression.
Finding molar solubility from Ksp
The common ion effect
Adding an ion that's already part of the equilibrium, for example adding NaCl to a saturated AgCl solution, shifts the dissolving equilibrium back toward the solid, decreasing how much AgCl can dissolve. This is simply Le Chatelier's Principle applied to a solubility equilibrium.
Why you can't always compare Ksp values directly
A larger Ksp only guarantees greater solubility when comparing two salts with the same ion ratio, like two different 1:1 salts. For salts with different formulas, like a 1:1 salt compared to a 1:2 salt, the actual molar solubility has to be calculated for each before a fair comparison can be made, since the exponents in the Ksp expression are different.
Where students actually lose points here
Forgetting the coefficient becomes an exponent
For a salt like PbI2, the expression is Ksp = [Pb2+][I−]², not [Pb2+][I−]. Missing the exponent from the formula's subscript gives a completely wrong relationship between Ksp and molar solubility.
Comparing Ksp values across different salt types directly
Without matching stoichiometry, a bigger Ksp doesn't necessarily mean bigger solubility. Each has to be converted to actual molar solubility first.
Getting the common ion effect direction backward
Adding a common ion decreases solubility, it doesn't increase it, since it shifts the dissolving equilibrium back toward the undissolved solid.
Equilibrium ideas applied to solubility
Once Ksp is seen as just another equilibrium constant, this topic stops feeling like a brand new set of rules and becomes a direct extension of everything from earlier in Unit 7.
Common questions
Does adding a common ion increase or decrease solubility?
It decreases solubility. Adding an ion that's already part of the equilibrium shifts the dissolving equilibrium back toward the undissolved solid, which is Le Chatelier's Principle applied to solubility.
Can you compare Ksp values directly to predict which salt is more soluble?
Only when both salts have the same ion ratio, like two 1:1 salts. For salts with different formulas, molar solubility has to be calculated separately for each before comparing.
How do you find molar solubility from Ksp?
Set up the Ksp expression in terms of a variable representing molar solubility, using the stoichiometry of the dissolving reaction to determine each ion's concentration in terms of that variable, then solve.