As you know, ionic solids dissolves in a polar solvent (Like Dissolves Like). As shown in the animation from SolutionBookNotes1 handout ([click here to go to website (by mhhe) that animates this process]), and ([Notes on Which Substance Dissolve Miscible in Another Substance] webpage and You Tube video on Dissolve and Miscible), the ionic solid will dissolve into its ions until such time that the given solvent volume can not fit any more solute in it. |
As you know, ionic solids dissolves in a polar solvent (Like Dissolves Like). As shown in the animation from SolutionBookNotes1 handout ([click here to go to website (by mhhe) that animates this process]), and ([Notes on Which Substance Dissolve Miscible in Another Substance] webpage and You Tube video on Dissolve and Miscible, the ionic solid will dissolve into its ions until such time that the given solvent volume can not fit any more solute in it. |
Therefore, the solution is said to be saturated or it is at its saturation point. The rate of the ionic solid dissolving is equal to rate of ions going back into solid phase. There is a dynamic EQUILIBRIUM established. This is called Solubility Equilibria which is treated like any other equilibrium problems.
Please remember that adding less than the amount of solute to get to the saturation point, the solution is called unsaturated and adding more is called a saturated solution with solid (stuff) in the bottom of the bucket. It is not a supersaturated solution. Supersaturation requires a lot of work (increase temp then saturate the solution and let cool down, etc).
At the saturation point, the concentration (amount) of the solid that has dissolved into its ions is called the solubility (sometimes molar solubility) of that ionic solid. Of course, the equilibrium concentration of ions that have formed in the solution is determined using the stoichiometry of the reaction (it is really not a chemical reaction but a physical process however it behavior the same with respect to the calculations).
In general there are usually only two types of questions first year students will be asked.
a. Given the Ksp of the solid, calculating the molar solubility of the solid (and/or the equilibrium concentrations of the ions.)
The trick here is emphasized in the video (so look at your notes) where he uses "x" as solubility of solid and determines the equilibrium concentration of the ions by the stoichiometry of the reaction.
b. Given the molar solubility of the solid (and/or equilibrium concentration of ions), determine the Ksp of the solid.
There is no trick here, you just use stoichiometry to determine the equilibrium concentration of the ions and plug into the Ksp expression. I would like you to show how you used the molar ratio to determine the equilibrium concentration of the ions, though.