Calculate the pH of a Saturated Solution of Ca(OH)2
Use this premium chemistry calculator to estimate the pH, pOH, hydroxide concentration, calcium ion concentration, and molar solubility of saturated calcium hydroxide from its solubility product constant.
Ca(OH)2 Saturation Calculator
Calculation Visualization
This chart compares the major calculated quantities for the saturated calcium hydroxide solution.
s = (Ksp / 4)^(1/3)
[OH-] = 2s
pOH = -log10([OH-])
pH = pKw – pOH
Expert Guide: How to Calculate the pH of a Saturated Solution of Ca(OH)2
Calcium hydroxide, written as Ca(OH)2, is a classic example of a sparingly soluble strong base that appears in general chemistry, analytical chemistry, environmental chemistry, and industrial water treatment. If you need to calculate the pH of a saturated solution of Ca(OH)2, the key idea is that the compound does not dissolve completely in unlimited quantity. Instead, it establishes a solubility equilibrium with water. That equilibrium is governed by the solubility product constant, commonly written as Ksp.
Students often know that hydroxides are basic, but the pH of a saturated solution is not found by assuming an arbitrary concentration. It must be determined from equilibrium chemistry. In practice, you write the dissolution equation, define the molar solubility, express the equilibrium concentrations in terms of that molar solubility, solve for the hydroxide concentration, then convert to pOH and finally to pH. The calculator above automates those steps, but understanding the method is what makes the result meaningful.
Step 1: Write the dissolution equilibrium
For calcium hydroxide in water, the dissolution process is:
Ca(OH)2(s) ⇌ Ca2+(aq) + 2OH-(aq)
This equation tells you two important stoichiometric facts:
- Each mole of dissolved Ca(OH)2 creates 1 mole of Ca2+.
- Each mole of dissolved Ca(OH)2 creates 2 moles of OH–.
If the molar solubility is s, then at equilibrium:
- [Ca2+] = s
- [OH-] = 2s
Step 2: Write the Ksp expression
The solid Ca(OH)2 does not appear in the equilibrium expression because pure solids have constant activity. The solubility product therefore becomes:
Ksp = [Ca2+][OH-]^2
Substitute the stoichiometric relationships from the dissolution equation:
Ksp = (s)(2s)^2 = 4s^3
From that point, solve for s:
s = (Ksp / 4)^(1/3)
Once you know s, the hydroxide concentration follows immediately:
[OH-] = 2s
Step 3: Convert hydroxide concentration to pOH and pH
Because calcium hydroxide is a strong base once dissolved, the dissolved hydroxide ions directly determine basicity. Use the standard logarithmic definition:
pOH = -log10([OH-])
Then compute pH from:
pH = 14.00 – pOH
At ordinary classroom conditions, the value 14.00 is usually used for pH + pOH. In more advanced work, especially when temperature differs significantly from 25 degrees Celsius, your instructor may ask you to use a different pKw. That is why the calculator includes an optional custom pKw field.
Worked example using a common Ksp value
Suppose the problem gives Ksp = 5.5 × 10^-6 for Ca(OH)2 at 25 degrees Celsius. We proceed as follows:
- Start from Ksp = 4s^3.
- Solve for molar solubility: s = (5.5 × 10^-6 / 4)^(1/3).
- This gives s ≈ 0.0111 M.
- Then [OH-] = 2s ≈ 0.0222 M.
- Now calculate pOH = -log10(0.0222) ≈ 1.653.
- Finally, pH = 14.000 – 1.653 = 12.347.
So the pH of a saturated solution of calcium hydroxide is approximately 12.35 when using this common Ksp value and the standard 25 degree Celsius assumption that pH + pOH = 14.00.
Why the pH is high but not as high as a concentrated strong base
Many learners hear that calcium hydroxide is a strong base and then expect a pH very close to 14. The important distinction is between strength and solubility. Strength refers to how completely a dissolved species dissociates. Solubility refers to how much of that substance can dissolve in water. Ca(OH)2 is strongly dissociated once dissolved, but it is only moderately soluble compared with highly soluble bases like sodium hydroxide. That limited solubility caps the hydroxide concentration and prevents the pH from reaching the values you would see in concentrated NaOH or KOH solutions.
Common mistake to avoid
- Forgetting the coefficient 2 on OH–: Since each dissolved formula unit produces two hydroxide ions, the hydroxide concentration is 2s, not s.
- Writing Ksp incorrectly: The correct expression is [Ca2+][OH-]^2, not just [Ca2+][OH-].
- Confusing pOH with pH: Calculate pOH first from hydroxide concentration, then convert to pH.
- Using the wrong stoichiometric relationship: Calcium hydroxide does not dissolve into one calcium and one hydroxide. It dissolves into one calcium and two hydroxides.
- Ignoring temperature assumptions: Ksp changes with temperature, and pKw may also change, so different sources can produce slightly different pH results.
Comparison table: Ca(OH)2 saturation chemistry at several Ksp values
| Ksp for Ca(OH)2 | Molar solubility, s (M) | [OH-] at saturation (M) | pOH | pH at pKw = 14.00 |
|---|---|---|---|---|
| 4.68 × 10^-6 | 0.01054 | 0.02108 | 1.676 | 12.324 |
| 5.50 × 10^-6 | 0.01112 | 0.02224 | 1.653 | 12.347 |
| 6.50 × 10^-6 | 0.01176 | 0.02351 | 1.629 | 12.371 |
This comparison shows a useful practical point: even though literature values for Ksp differ slightly from source to source, the calculated pH for a saturated calcium hydroxide solution usually lands in a fairly narrow range near 12.3 to 12.4 under standard assumptions. That is why many textbook examples converge on a value around pH 12.4.
Mass concentration and practical interpretation
In laboratory or field settings, it is often helpful to translate molar solubility into a mass concentration. The molar mass of Ca(OH)2 is about 74.09 g/mol. If s ≈ 0.0111 M, then the dissolved mass concentration is:
0.0111 mol/L × 74.09 g/mol ≈ 0.823 g/L
That value aligns with the general understanding that calcium hydroxide is only slightly soluble in water compared with highly soluble alkali hydroxides. Yet because each dissolved unit releases two hydroxide ions, the resulting solution is still strongly basic.
Comparison table: Saturated Ca(OH)2 versus common strong base solutions
| Solution | Approximate [OH-] (M) | Approximate pOH | Approximate pH | Key reason |
|---|---|---|---|---|
| Saturated Ca(OH)2 | 0.022 | 1.65 | 12.35 | Strong base but limited by solubility equilibrium |
| 0.10 M NaOH | 0.10 | 1.00 | 13.00 | Fully dissolved and highly soluble |
| 1.00 M NaOH | 1.00 | 0.00 | 14.00 | Very high hydroxide concentration |
This contrast helps clarify why a saturated solution of limewater is strongly basic but still noticeably less basic than concentrated sodium hydroxide. Solubility matters every bit as much as dissociation when you are predicting pH.
What if there is a common ion present?
The simple saturated-solution calculation assumes pure water and no competing equilibria. If a source of calcium ions or hydroxide ions is already present, the common ion effect reduces the solubility of Ca(OH)2. In that case, the equilibrium concentrations are no longer described by the pure-water relation alone. For instance, if CaCl2 is present, the extra Ca2+ suppresses dissolution. If NaOH is present, the extra OH– also suppresses dissolution. In both scenarios, less Ca(OH)2 dissolves than in pure water.
That means the calculator on this page is designed for the standard problem statement: calculate the pH of a saturated solution of calcium hydroxide in water. If your assignment includes a common ion, you need a different equilibrium setup.
Why literature values can vary
You may notice that textbooks, lab manuals, and online chemistry resources do not always use the exact same Ksp for calcium hydroxide. That is normal. Reported values may differ because of temperature, ionic strength, data rounding, or source conventions. In educational settings, it is best to use the value supplied in the problem statement. If no value is supplied, a common classroom approximation is 5.5 × 10^-6 at 25 degrees Celsius.
Authority sources for chemistry equilibrium and water chemistry
For deeper reading, consult authoritative chemistry and water-quality resources:
- U.S. Environmental Protection Agency water research resources
- Chemistry LibreTexts educational chemistry library
- NIST Chemistry WebBook
Fast exam strategy
If you are solving this under test conditions, remember this compact path:
- Write Ca(OH)2 ⇌ Ca2+ + 2OH-.
- Set [Ca2+] = s and [OH-] = 2s.
- Use Ksp = 4s^3.
- Find s = (Ksp/4)^(1/3).
- Compute [OH-] = 2s.
- Calculate pOH = -log[OH-].
- Convert to pH = 14 – pOH.
That is the entire logic of the problem. Once you master this workflow, you can handle similar calculations for Mg(OH)2, BaF2, Ag2CO3, and other sparingly soluble ionic compounds.
Final takeaway
To calculate the pH of a saturated solution of Ca(OH)2, you do not guess the concentration. You derive it from the solubility equilibrium. The core relation is Ksp = 4s^3, because each dissolved unit contributes one calcium ion and two hydroxide ions. After solving for s, you obtain hydroxide concentration, then pOH, then pH. Using a common 25 degree Celsius value of Ksp = 5.5 × 10^-6, the saturated solution has a pH of about 12.35. The result is high because the dissolved portion is a strong base, but it remains below the pH of concentrated alkali hydroxides because calcium hydroxide is only sparingly soluble.