Solubility Equilibrium: Ksp, Common-Ion Effect & Precipitation
Calculate Ksp and predict precipitation
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Solubility Equilibrium: Ksp, Common-Ion Effect & Precipitation
A "sparingly soluble" ionic solid placed in water reaches a dynamic equilibrium between dissolution and precipitation. We describe that equilibrium with a special equilibrium constant called (the solubility product). Mastering lets you (1) calculate molar solubility, (2) predict whether a precipitate will form when two solutions are mixed, (3) explain why solubility drops in the presence of a common ion, (4) explain why pH affects the solubility of certain salts, and (5) design selective precipitation schemes — all skills tested directly on the AP Chemistry exam (Unit 7 equilibrium and Unit 8 acid–base extensions).
1. Expressions — the foundation
For a generic salt ,
Pure solids and pure liquids do not appear in the expression. Examples:
- :
- :
- :
The smaller the , the less soluble the salt. But you can only directly compare solubilities of two salts by if they have the same stoichiometry — otherwise compute molar solubility first.
2. Molar Solubility from
Let = molar solubility (mol/L of salt that dissolves to saturate the solution).
For : , → , so .
For : , → , so .
For : .
AP tip. Always start with the dissolution equation, then read off the stoichiometric coefficients to set the ion concentrations in terms of . Most " vs. solubility" wrong answers come from forgetting the coefficient (, not ) or failing to raise to the right power.
3. The Common-Ion Effect
By Le Châtelier's principle, adding an ion already present in the dissolution equilibrium shifts the equilibrium back toward the solid, decreasing solubility.
Example. . In pure water, M.
In 0.10 M NaCl, is essentially fixed at 0.10 M (the trace from AgCl is negligible). So The molar solubility crashes from M to M — about four orders of magnitude lower.
The common-ion effect underlies why "salting out" works in many separations.
4. vs. — Will a Precipitate Form?
The reaction quotient for a dissolution uses the same expression as but plugged with current (not equilibrium) ion concentrations.
| Comparison | Meaning |
|---|---|
| Unsaturated. More solid can dissolve. No precipitate. | |
| Saturated equilibrium. Solid and dissolved ions in balance. | |
| Supersaturated. Precipitate forms until falls back to . |
Worked example. Mix 50 mL of 0.0010 M with 50 mL of 0.020 M NaI. .
After mixing (volumes add → each is diluted by ½):
- M
- M
Since , PbI₂ precipitates.
5. pH Effects on Solubility
A salt becomes more soluble in acidic solution if its anion is a conjugate base of a weak acid (or if its cation reacts with OH⁻ in basic solution).
Examples.
- is more soluble in acidic solution. The ion reacts with to form HF (a weak acid), removing from solution and pulling the dissolution equilibrium right.
- is more soluble in acidic solution (acid neutralizes OH⁻) and less soluble in basic solution (added OH⁻ is a common ion).
- solubility is not noticeably affected by pH because is the conjugate base of a strong acid (HCl) and doesn't react with .
Rule of thumb. Salts of weak-acid anions (F⁻, OH⁻, S²⁻, , , …) become more soluble at low pH. Salts of strong-acid anions (Cl⁻, Br⁻, I⁻, , ) are essentially pH-independent.
6. Selective Precipitation
When a solution contains two ions that both form an insoluble salt with a common reagent, the one with the smaller precipitates first (assuming similar stoichiometry).
Example. A solution is 0.10 M in both and . Slowly add AgNO₃. Use and .
- AgI starts to precipitate when M.
- AgCl starts when M.
AgI begins to precipitate at a much lower [Ag⁺]. By the time AgCl just begins to precipitate, has been reduced to M — i.e., over 99.99999% of the iodide has been removed before any chloride precipitates. This is the basis of qualitative-analysis cation/anion separation schemes.
7. Synthesis & AP Review
Big ideas to leave with:
- is just an equilibrium constant for the dissolution of a sparingly soluble salt. Solid omitted, ions to their stoichiometric powers.
- Always relate molar solubility to via the dissolution stoichiometry (e.g., for ).
- The common-ion effect dramatically lowers solubility (Le Châtelier on the dissolution equilibrium).
- Compare to to decide if a precipitate forms. Don't forget to dilute ion concentrations after mixing.
- Solubility of salts with weak-acid anions increases at low pH; salts of strong-acid anions are essentially pH-independent.
- Selective precipitation exploits differences in to separate ions.
Common mistakes
- Forgetting to raise ion concentrations to the stoichiometric power in (, not , for ).
- Using molar solubility as a value, or vice versa.
- Forgetting to dilute when mixing two solutions before computing .
- Comparing values of salts with different stoichiometries to rank solubilities. Compute first.
- Saying that pH affects all salts. It only matters when the cation reacts with OH⁻ or the anion reacts with H⁺.
Quick reference card
- for :
- for or :
- for or :
- for :
- Common ion → solubility ↓ (Le Châtelier)
- → precipitate forms
- Weak-acid anion (F⁻, OH⁻, , …) → solubility ↑ at low pH
📚 Practice Problems
1Problem 1easy
❓ Question:
Write the dissolution equation and the expression for silver chloride, .
💡 Show Solution
Dissolution: .
The pure solid is omitted from the equilibrium expression:
At 25 °C, .
2Problem 2easy
❓ Question:
Write the expression for calcium phosphate, .
💡 Show Solution
Dissolution: .
Note both the coefficients (3 cations, 2 anions per formula unit) and the corresponding exponents.
3Problem 3easy
❓ Question:
For , at 25 °C. Calculate the molar solubility in pure water.
💡 Show Solution
.
Let = molar solubility. Then and :
→ .
4Problem 4medium
❓ Question:
The of is . Calculate its molar solubility in pure water.
💡 Show Solution
.
ICE: , .
→ .
A notably more-soluble salt than the silver halides above — consistent with being roughly times larger.
5Problem 5medium
❓ Question:
Explain the common-ion effect. Quantitatively, by what factor does the molar solubility of () drop on going from pure water to 0.10 M NaCl?
💡 Show Solution
Concept. Adding an ion already in the dissolution equilibrium shifts the equilibrium back to solid (Le Châtelier), reducing solubility.
In pure water: M.
In 0.10 M NaCl: M (the trace from AgCl is negligible). Then
M.
now equals the molar solubility, so M.
Factor: .
6Problem 6medium
❓ Question:
100 mL of M is mixed with 100 mL of M NaCl. . Will a precipitate form?
💡 Show Solution
Step 1. After mixing, total volume = 200 mL, so each species is diluted by a factor of 2:
M, M.
Step 2. .
Step 3. , so until falls back to .
7Problem 7medium
❓ Question:
Why is much more soluble in 0.10 M HCl than in pure water, but essentially insoluble in 0.10 M NaOH?
💡 Show Solution
Dissolution: .
In 0.10 M HCl (acid): neutralizes (), removing a product. Le Châtelier shifts dissolution right, dramatically increasing solubility.
In 0.10 M NaOH (base): is a common ion at 0.10 M. The equilibrium shifts left, suppressing dissolution. Solubility crashes.
Quantitatively, in 0.10 M NaOH, M (essentially nothing dissolves).
8Problem 8hard
❓ Question:
A solution is 0.010 M in both and . Solid is added slowly with stirring. (; .) (a) Which precipitates first? (b) When the second salt just begins to precipitate, what is ? Has separation been effective?
💡 Show Solution
(a) AgI starts when M.
AgCl starts when M.
AgI requires far less , so .
(b) When AgCl just begins to precipitate, M. At that point
.
Fraction remaining: , i.e. >99.99995% of the iodide has been removed before any chloride precipitates. Excellent separation.
9Problem 9hard
❓ Question:
Calculate the molar solubility of () in (a) pure water, and (b) 0.010 M .
💡 Show Solution
Dissolution: .
(a) Pure water. Let = molar solubility. Then , .
→ → .
(b) 0.010 M (common ion = ). Now M (the trace from is negligible). Let new molar solubility, so .
→ .
The common ion drops solubility by ~12×.
10Problem 10hard
❓ Question:
has . (a) Calculate its molar solubility in pure water. (b) Qualitatively explain why is more soluble in 0.10 M HCl than in pure water, but does not show such a pH dependence.
💡 Show Solution
(a) . Let = molar solubility. Then , .
→ → .
(b) is the conjugate base of the weak acid HF (pKa ≈ 3.2). In 0.10 M HCl, the high converts much of the dissolved to undissociated HF, removing from the equilibrium. By Le Châtelier, dissolution shifts right and solubility increases substantially.
For , the anion is , the conjugate base of the strong acid HCl — it has essentially no tendency to combine with . So acid does not consume and the dissolution equilibrium is unaffected by pH. ( is also extremely soluble in water for this kind of analysis to apply, but the key point is the lack of pH coupling.)
📋 AP Chemistry — Exam Format Guide
| Section | Format | Questions | Time | Weight | Calculator |
|---|---|---|---|---|---|
| Multiple Choice | MCQ | 60 | 90 min | 50% | ✅ |
| Free Response (Long) | FRQ | 3 | 69 min | 30% | ✅ |
| Free Response (Short) | FRQ | 4 | 36 min | 20% | ✅ |
📊 Scoring: 1-5
💡 Key Test-Day Tips
- ✓Memorize common polyatomic ions
- ✓Practice dimensional analysis
- ✓Know your gas laws
⚠️ Common Mistakes: Solubility Equilibrium: Ksp, Common-Ion Effect & Precipitation
Avoid these 3 frequent errors
🌍 Real-World Applications: Solubility Equilibrium: Ksp, Common-Ion Effect & Precipitation
See how this math is used in the real world
📝 Worked Example: Stoichiometry — Limiting Reagent
mol of reacts with mol of . How many grams of water are produced? Which is the limiting reagent? ()
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