Name: _________________________ Solubility and Ksp

Many ionic compounds are very soluble (like those with Na+, K+, NH4+ and
¡ Importante ! Every step with the 
NO3-) ions. Other ionic compounds are only slightly soluble and will tend to symbol requires a written answer.
form saturated solutions that also contain undissolved solid. A dynamic Show your work where appropriate.
equilibrium is established between the solid form of the compound and its ions
in aqueous solution. For example, if ionic compound XY2 forms a saturated solution, the reversible process is:
XY2 (s)  X2+ (aq) + 2 Y- (aq)
The equilibrium constant, K, in this case is Ksp, where sp stands for “solubility product”, is written like this:
Ksp = [X2+][Y-]2
Note that the denominator is 1, because the “reactant” in this case is a solid, so there is no concentration.
The concentration of each of the reactants is the maximum amount that can dissolve in the given amount of
solvent, so we call that the “solubility” of the ions.
In this simulation, you’ll create two saturated solutions, then calculate the concentrations of each of the ions in
the compound. You’ll use those concentrations to estimate the K sp of the ionic compound.
1.Here’s the link: https://phet.colorado.edu/sims/cheerpj/soluble-salts/latest/soluble-salts.html?simulation=soluble-salts
2.Select the tab for “Slightly Soluble Salts”, then select “silver bromide” as the first salt you’ll investigate.
3.Use the lower faucet to drain out water until the level is approximately 5.0 x 10-17
liters. Don’t worry about getting it exact, because the simulation will tell you exactly
what the volume of water is.
4.Give the shaker a shake to add solid AgBr into the water. Make sure that the compound is completely dissolved. The
“bound molecules” should be 0. If you have undissolved solid, use the down buttons next to the “total” line to remove
some of it.
mol
5. Calculate the concentration ( ) of Ag+ and Br- ions in the solution. You will need to
L
use Avogadro’s number, because you are starting with ions and converting them to moles!

6.  Shake the shaker a few more times until it’s clear that some of the solid is not dissolving. There should be some
“bound molecules” at this point. Would you describe this solution as saturated or unsaturated? Look carefully at the
particles near the solid. What evidence do you see that this is dynamic equilibrium?

7.  Calculate the concentration (mol/L) of the Ag+ ions and Br- ions at saturation. The numbers of ions will fluctuate a
little, because this is a dynamic equilibrium (the particles continue moving from (s) to (aq) and vice versa.), but they
should be relatively stable.

These concentrations represent the solubility of those ions. You cannot dissolve any more than that concentration.
These concentrations should be higher than the concentrations you found in the unsaturated solution in Step 5.
8.  Write the net ionic equation for the reversible process between solid AgBr and its aqueous ions in a saturated
solution.

9.  Write the equilibrium expression, Ksp,

10.  Calculate the Ksp for AgBr, using the

On to part 2!
11. Reset the simulation. Drain the water so that there is somewhere between 4.0 x 10 -17 L and 5.0 x 10-17 L.
 Record the actual volume of water:

12. Select Strontium Phosphate (Sr3(PO4)2 as the solute. Shake the salt into the water until it is just saturated.

13. Record the dissolved ions for both Strontium and Phosphate. The numbers will fluctuate a little, because this is a
dynamic equilibrium so the particles continue moving from (s) to (aq) and vice versa.
Sr2+ : PO43- :

mol
14.  Calculate the concentration ( L ) of each ion at saturation. Remember, these are ions, so you’ll need Avogadro’s
number.

15.  Write the net ionic equation for this process of dissolving Strontium Phosphate:

16.  Write the Ksp expression for

17.  Calculate the Ksp of Strontium Phosphate using

That’s it! In class we will cover going backwards – finding the molar solubility of ions if we know the K sp of a
given ionic compound.