Calculate the pH of 0.120 M Ca(OH)2
Use this interactive calculator to find hydroxide concentration, pOH, and pH for calcium hydroxide solutions. The default example solves the exact problem: calculate the pH of 0.120 M Ca(OH)2.
Calculated Results
Enter or keep the default values, then click Calculate pH.
Solution Profile Chart
This chart compares the original base concentration, resulting hydroxide concentration, pOH, and pH for the selected Ca(OH)2 solution.
Ca(OH)2 → Ca2+ + 2OH–
[OH–] = 2 × molarity of Ca(OH)2
pOH = -log[OH–]
pH = 14 – pOH
How to calculate the pH of 0.120 M Ca(OH)2
To calculate the pH of 0.120 M calcium hydroxide, begin by recognizing that calcium hydroxide is a strong base that dissociates substantially in standard textbook pH problems. Each formula unit of Ca(OH)2 produces one calcium ion and two hydroxide ions. That stoichiometric ratio is the key to the entire problem. If the calcium hydroxide concentration is 0.120 M, then the hydroxide concentration is twice that value, which gives [OH–] = 0.240 M. Once hydroxide concentration is known, calculate pOH using the base-10 logarithm, then convert pOH to pH using the relationship pH + pOH = 14.00 at 25 degrees Celsius.
The exact sequence is straightforward:
- Write the dissociation equation: Ca(OH)2 → Ca2+ + 2OH–.
- Multiply the base molarity by 2 because each mole of calcium hydroxide yields two moles of OH–.
- Compute pOH = -log(0.240).
- Compute pH = 14.00 – pOH.
Doing the math gives a pOH of about 0.620, and therefore the pH is about 13.380. This is exactly what you should expect for a moderately concentrated strong base. Since the hydroxide concentration is high, the pOH is low, and because pH is the complement of pOH on the 14-point scale at 25 degrees Celsius, the pH ends up strongly basic.
Step-by-step worked example
1. Write the chemical dissociation
Calcium hydroxide dissociates as follows:
Ca(OH)2 → Ca2+ + 2OH–
This equation tells you that every 1 mole of dissolved Ca(OH)2 contributes 2 moles of hydroxide ions. Students often make the mistake of using 0.120 M directly as [OH–], but that would ignore the stoichiometric coefficient of 2.
2. Determine hydroxide concentration
If the calcium hydroxide concentration is 0.120 M, then:
[OH–] = 2 × 0.120 = 0.240 M
3. Convert hydroxide concentration to pOH
Use the formula:
pOH = -log[OH–]
Substitute the value:
pOH = -log(0.240) ≈ 0.620
4. Convert pOH to pH
At 25 degrees Celsius, pH and pOH are related by:
pH + pOH = 14.00
So:
pH = 14.00 – 0.620 = 13.380
Final answer: The pH of 0.120 M Ca(OH)2 is 13.38 when complete dissociation is assumed at 25 degrees Celsius.
Why calcium hydroxide changes pH so strongly
Calcium hydroxide is often called slaked lime or hydrated lime. In water, it acts as a strong base because it contributes hydroxide ions directly. In acid-base chemistry, pH depends on the concentration of hydrogen ions or, indirectly for bases, on the concentration of hydroxide ions. A substance that releases more hydroxide per mole has a larger impact on pH than a base with the same molarity but fewer OH– ions per formula unit.
That is why 0.120 M Ca(OH)2 is more basic than 0.120 M NaOH on a hydroxide basis. Sodium hydroxide provides one OH– per formula unit, so 0.120 M NaOH gives 0.120 M OH–. In contrast, 0.120 M Ca(OH)2 gives 0.240 M OH–. This difference has a measurable effect on pOH and pH.
| Base | Formal Concentration (M) | OH- per Formula Unit | Resulting [OH-] (M) | pOH | pH at 25 C |
|---|---|---|---|---|---|
| NaOH | 0.120 | 1 | 0.120 | 0.921 | 13.079 |
| Ca(OH)2 | 0.120 | 2 | 0.240 | 0.620 | 13.380 |
| Ba(OH)2 | 0.120 | 2 | 0.240 | 0.620 | 13.380 |
Important chemistry concepts behind the calculation
Molarity
Molarity is moles of solute per liter of solution. A 0.120 M solution contains 0.120 moles of calcium hydroxide per liter of solution. Molarity alone does not always equal the concentration of the ions that matter in pH calculations. You must also consider dissociation stoichiometry.
Stoichiometric coefficient of hydroxide
In the formula Ca(OH)2, there are two hydroxide groups attached to calcium. When the compound dissociates, those two hydroxide ions appear in solution. Therefore, hydroxide concentration is doubled relative to the formula molarity under complete dissociation assumptions.
pOH and logarithms
pOH is a logarithmic measure, not a linear one. A small numerical change in pOH represents a significant change in hydroxide concentration. That is why it is important to calculate [OH–] correctly before taking the logarithm. If you skip the coefficient of 2, your pOH and pH values will both be wrong.
The 14.00 relationship
In many general chemistry problems at 25 degrees Celsius, we use:
pH + pOH = 14.00
This relationship comes from the ionic product of water, Kw = 1.0 × 10-14. At other temperatures, the value changes slightly, but for standard educational problems the 14.00 relation is correct and expected.
Common mistakes students make
- Forgetting the coefficient 2: Using [OH–] = 0.120 M instead of 0.240 M.
- Using natural log instead of base-10 log: pOH and pH use log base 10.
- Subtracting from 7 instead of 14: The proper conversion is pH = 14 – pOH at 25 C.
- Confusing strong and weak bases: Introductory pH problems generally treat Ca(OH)2 as fully dissociated once dissolved.
- Rounding too early: Keep extra digits during the calculation, then round at the end.
How this compares with real pH reference ranges
The pH value of 13.38 places this solution in a highly basic range. For context, standard environmental and drinking water guidelines are far lower. Natural waters, treated drinking water, and many industrial process waters usually exist in much narrower pH windows than concentrated laboratory base solutions.
| Water or Solution Category | Typical or Recommended pH Range | Source Type | How 0.120 M Ca(OH)2 Compares |
|---|---|---|---|
| U.S. drinking water secondary standard | 6.5 to 8.5 | EPA guidance | Much more basic than recommended drinking water range |
| Common freshwater systems | About 6.5 to 9.0 | Environmental monitoring references | Far above normal natural water pH |
| 0.120 M Ca(OH)2 | 13.38 | Stoichiometric pH calculation | Extremely basic relative to water quality ranges |
Practical applications of calcium hydroxide
Calcium hydroxide has many real-world uses. It appears in water treatment, soil stabilization, flue gas treatment, food processing under regulated conditions, and chemical manufacturing. Because it raises pH strongly, it can neutralize acidic streams or precipitate metal ions in wastewater treatment systems. In civil engineering and environmental chemistry, understanding how Ca(OH)2 affects pH is crucial for process control and safety.
In educational settings, calcium hydroxide also appears in classic limewater experiments. Limewater can be used to test for carbon dioxide because CO2 reacts with dissolved calcium hydroxide to form calcium carbonate, producing a cloudy appearance. Even though that is a different topic from a direct pH calculation, it reinforces the idea that Ca(OH)2 contributes OH– ions and participates in acid-base reactions very readily.
When the simple textbook approach is appropriate
For a problem stated as “calculate the pH of 0.120 M Ca(OH)2,” the expected general chemistry approach is to assume the dissolved base dissociates completely and to ignore activity corrections. This is the correct method for homework, quizzes, exams, and many introductory calculators. In advanced physical chemistry or analytical chemistry, a more rigorous treatment might include ionic strength, non-ideal behavior, or solubility limitations under certain conditions. But unless the problem explicitly asks for those effects, the standard strong-base method is the right one.
Authoritative references for pH and water chemistry
If you want to verify pH fundamentals and water quality ranges from trusted institutions, these resources are excellent starting points:
- U.S. Environmental Protection Agency: pH overview and environmental significance
- U.S. Geological Survey: pH and water science basics
- LibreTexts chemistry materials used by colleges and universities
Quick summary formula for this exact problem
For this specific concentration:
- Given: [Ca(OH)2] = 0.120 M
- Find hydroxide: [OH–] = 2(0.120) = 0.240 M
- Find pOH: pOH = -log(0.240) = 0.620
- Find pH: pH = 14.000 – 0.620 = 13.380
If you remember only one thing, remember this: because Ca(OH)2 releases two hydroxide ions, always double the molarity before calculating pOH. That single step leads directly to the correct answer.