LESSON 26.1
Tri-Valley AP Chemistry
C60
THE TRUTH ABOUT SOLUBILITY
Opportunity + Dedication  Success
CAN YOU HANDLE THE TRUTH?
Part 0: An Introduction to Solubility
Our 5 solubility rules, though generally quite useful to predict if a precipitate will form or if a compound will
dissolve, are sometimes just too simple. In reality, EVERYTHING dissolves in water to some extent; all that
differs is the extent of solubility.
For example, if you’re dissolving NaCl in water, expect to add about 6.3 moles (370 g) per liter of water to
create a saturated solution (a solution in which no more solute can be dissolved). If you’re dissolving AgCl,
however (which our solubility rules predict to be insoluble), you’ll only need 0.000126 moles (0.00180 g)
per liter of water to saturate the solution. That’s a big difference.
Like all processes, dissolving is governed by equilibrium, and so we can write equilibrium expressions and
calculate equilibrium constants (called Ksp) for dissolving processes.
Example 1: Write the equilibrium expressions for the following “reactions”:
NaCl(s) ↔ Na+(aq) + Cl-(aq)
Ksp =
PbCl2(s) ↔ Pb+2(aq) + 2 Cl-(aq)
Ksp =
As you can see, the presence of solids greatly simplifies the equilibrium expressions, as well as the
calculations…
Before we continue, we must define two similar words:
Solubility – The amount of stuff that can dissolve in a given amount of solvent at a given
temperature. Units are often mols/L or g/L.
Solubility Product – The equilibrium constant (Ksp) for a given dissolution at a given temperature.
Ksp is generally unitless.
Part 1: Interpreting Ksp Values
Example 2. The Ksp of PbCl2 is 1.6 × 10-5 and the Ksp of Pb(OH)2 is 1.2 × 10-15.
a) Write reactions that correspond to these K values.
b) Which is more soluble in water, PbCl2 or Pb(OH)2? Explain.
Example 3. Lead is a neurotoxin and is highly regulated. Imagine you have a solution of lead nitrate that
you would like to dispose of in a landfill (don’t do it!). If you wanted to minimize the amount of lead
dissolving and possibly contaminating the water supply, which lead precipitate should you form? Explain.
a) Pb(NO3)2
b) PbSO4 (Ksp = 1.3 × 10-8)
d) PbCO3 (Ksp = 1.5 × 10-15)
e) PbS (Ksp = 7 × 10-29)
c) PbCrO4 (Ksp = 2 × 10-16)
Part 2: Solving Solubility Problems
There are really only two types of solubility problems that you can face: one asks you to solve for solubility,
the other for Ksp.
Example 4: At a given temperature, 0.0000126 mols AgCl(s) dissolve in 1.0 L of water. What is the Ksp of
AgCl at this temperature?
Step 1: Try an ICE table.
Step 2: Write the Ksp expression.
Step 3: Plug n’ chug.
Example 5: The solubility of Li2CO3 at a given temperature is 0.074 mol/L. What is the value of Ksp for Li2CO3
at this temperature?
Step 1: Try an ICE table.
Step 2: Write the Ksp expression.
Step 3: Plug n’ chug.
Example 6: Go the other way! Solid PbI2 has a famous school-bus-yellow color. The Ksp of PbI2 is 1.4 × 10-8.
What is the concentration of Pb2+ and I- ions in a saturated solution of PbI2?
Example 7: Solid lead hydroxide is a white, crystalline compund. The Ksp of Pb(OH)2 is 1.2 × 10-15. Calculate
[Pb2+] and [OH-] in a saturated solution of Pb(OH)2.
What will happen if you add OH- to a saturated solution of PbI2? Let’s try it!
Example 6: The Ksp of PbCl2 (MM = 278 g/mol) is 1.6 × 10-5. What is its solubility, in units of mols/L and g/L?
Example 7: The Ksp of Pb(OH)2 (MM = 241 g/mol) is 1.2 × 10-15. What is its solubility, in units of mols/L and
g/L?
LESSON 26.2
Tri-Valley AP Chemistry
C60
Opportunity + Dedication  Success
FACTORS THAT AFFECT SOLUBILITY
Part 1: When They’re Not Alone. Solubility depends on what else is in solution.
The solubility of compounds is affected by other species “cohabitating” with it in solution. The most
common cases are the “common ion effect” and acids or bases.
Example 1: The Common Ion Effect
The Ksp of AgCl at a given temperature is 1.6 × 10-10. Determine the solubility of AgCl in a 0.10 M solution of
NaCl. Compare the solubility of AgCl in this solution to the solubility in pure water (see Example 2).
Example 2: The Effect of Acids and Bases on pH
Magnesium hydroxide (also known as milk of magnesia) is often found in antacid tablets and liquids. The
solubility of Mg(OH)2 in pure water is 0.00013 mols/L at 25oC.
a) Write the reaction showing how Mg(OH)2(s) dissolves in water:
Will the solubility of Mg(OH)2 increase, decrease, or stay the same in the following solutions? Explain why.
b) 0.10 M NaOH
0.10 M HCl
Example 3: For each group of salts below, choose the salt that will dissolve the best in an acid solution.
Write equations for the reactions that occur with the salt to increase its solubility.
a) AgF, AgCl, AgBr
b) Pb(OH)2, PbCl2
Example 4: Turns out none of the suggested ways in yesterday’s Example 3 to dispose of lead are very good
ideas. Write equations showing how PbCO3 and PbS dissolve in acid solutions.