Calculate the pH of a 0.250 Solution of Sr(OH)2
This interactive chemistry calculator determines hydroxide concentration, pOH, and pH for aqueous strontium hydroxide solutions. Enter the molarity, choose your precision, and instantly see the dissociation math for this strong base.
Sr(OH)2 pH Calculator
Expert Guide: How to Calculate the pH of a 0.250 Solution of Sr(OH)2
To calculate the pH of a 0.250 solution of strontium hydroxide, written as Sr(OH)2, you use a short sequence of stoichiometry and logarithms. Because strontium hydroxide is a strong base, it dissociates essentially completely in water under ordinary general chemistry conditions. That means each dissolved mole of Sr(OH)2 produces one mole of Sr2+ and two moles of hydroxide ions, OH–. Once you know the hydroxide ion concentration, you calculate pOH from the negative base 10 logarithm, and then convert pOH to pH.
For a 0.250 M Sr(OH)2 solution, the central idea is simple: the hydroxide concentration is doubled because there are two hydroxide ions in the formula. Therefore:
- Start with the base concentration: 0.250 M Sr(OH)2.
- Multiply by 2 to account for the two hydroxide ions released per formula unit.
- Find pOH using pOH = -log[OH–].
- Find pH using pH = 14.00 – pOH at 25 C.
Step 1: Write the dissociation equation
The first step in any acid-base pH problem is to write the chemical equation that shows how the substance behaves in water. Strontium hydroxide dissociates according to:
Sr(OH)2(aq) → Sr2+(aq) + 2OH–(aq)
This equation tells you something crucial: one mole of Sr(OH)2 creates two moles of hydroxide ions. In pH calculations for strong bases, that stoichiometric factor can change the answer dramatically. Students often make the mistake of plugging in 0.250 M directly as [OH–], but that would ignore the coefficient of 2 in the balanced dissociation equation.
Step 2: Determine the hydroxide concentration
The molarity of the dissolved base is 0.250 M. Since each mole of Sr(OH)2 yields 2 moles of OH–, the hydroxide concentration is:
[OH–] = 2 × 0.250 = 0.500 M
This is the key conversion. Once you know [OH–], the chemistry becomes a logarithm problem rather than a stoichiometry problem. Because 0.500 M is a relatively high hydroxide concentration, you should expect a very high pH and a very low pOH.
Step 3: Calculate pOH
Use the standard definition:
pOH = -log[OH–]
Substitute the hydroxide concentration:
pOH = -log(0.500)
The common logarithm of 0.500 is about -0.3010, so:
pOH = 0.3010
It is normal for a concentrated strong base to have a pOH below 1. Since 0.500 M is a large hydroxide concentration, the pOH lands close to zero.
Step 4: Convert pOH to pH
At 25 C, the relationship between pH and pOH is:
pH + pOH = 14.00
So:
pH = 14.00 – 0.3010 = 13.699
Rounded suitably, the pH is 13.70. If your instructor expects three decimal places, you would report 13.699. The proper number of decimal places may depend on your course style guide or significant figure rules.
Why Sr(OH)2 is treated as a strong base
In most general chemistry and introductory analytical chemistry settings, Sr(OH)2 is treated as a strong base because the dissolved portion dissociates completely into ions. That puts it in the same classroom category as NaOH, KOH, and Ba(OH)2. The important difference from NaOH and KOH is that strontium hydroxide contains two hydroxide groups, so the hydroxide concentration is twice the formal concentration of the base.
That distinction matters. Compare 0.250 M NaOH with 0.250 M Sr(OH)2:
- 0.250 M NaOH gives 0.250 M OH–
- 0.250 M Sr(OH)2 gives 0.500 M OH–
As a result, a 0.250 M solution of strontium hydroxide is significantly more basic than a 0.250 M solution of sodium hydroxide if you compare them on the basis of hydroxide ion production.
Worked example in one line
If you want the fastest possible method, here it is:
0.250 M Sr(OH)2 → [OH–] = 0.500 M → pOH = -log(0.500) = 0.301 → pH = 14.00 – 0.301 = 13.699
Comparison table: strong bases at 0.100 M
The following table shows why stoichiometric ion release matters. These values assume complete dissociation at 25 C.
| Base | Formula concentration (M) | OH– released per mole | [OH–] (M) | pOH | pH |
|---|---|---|---|---|---|
| NaOH | 0.100 | 1 | 0.100 | 1.000 | 13.000 |
| KOH | 0.100 | 1 | 0.100 | 1.000 | 13.000 |
| Ca(OH)2 | 0.100 | 2 | 0.200 | 0.699 | 13.301 |
| Sr(OH)2 | 0.100 | 2 | 0.200 | 0.699 | 13.301 |
| Ba(OH)2 | 0.100 | 2 | 0.200 | 0.699 | 13.301 |
This comparison makes the pattern obvious: the number of hydroxide ions in the formula directly affects pOH and pH. For divalent metal hydroxides such as Sr(OH)2, the hydroxide concentration doubles relative to the formal molarity.
Temperature matters in more advanced work
In introductory chemistry, pH calculations commonly assume 25 C, where pH + pOH = 14.00. In more advanced treatment, that sum changes with temperature because the ion product of water, Kw, changes. If your class explicitly gives a different temperature or Kw value, you should use that instead of 14.00.
| Temperature | Approximate pKw | Interpretation |
|---|---|---|
| 0 C | 14.94 | Neutral water has pH above 7 |
| 25 C | 14.00 | Standard classroom condition |
| 50 C | 13.26 | Neutral water has pH below 7 |
Those are real thermodynamic trends discussed in standard chemistry references. The practical lesson is simple: for most homework on a 0.250 M Sr(OH)2 solution, assume 25 C unless your instructor says otherwise.
Common mistakes students make
- Forgetting the coefficient 2: Using 0.250 M instead of 0.500 M for [OH–].
- Calculating pH directly from base molarity: pH is not found from -log of the base concentration. You first find [OH–], then pOH, then pH.
- Mixing up pH and pOH: A strong base has high pH but low pOH.
- Using the wrong logarithm sign: pOH = -log[OH–], not log[OH–].
- Ignoring temperature assumptions: pH + pOH = 14.00 only applies exactly at 25 C.
How to check whether your answer is reasonable
A fast quality check can save points on exams. Ask yourself:
- Is Sr(OH)2 a base? Yes.
- Is 0.250 M fairly concentrated? Yes.
- Should the pH be above 7? Definitely.
- Should it be strongly basic, close to 14? Yes.
If you get something like pH 6.3 or pH 9.0, that is almost certainly wrong. A 0.250 M strong base that generates 0.500 M OH– must have a pH very near the upper end of the scale under standard conditions.
Why the pH is not exactly 14
Many people expect all strong bases to have pH = 14, but that is not true. The pH depends on the actual hydroxide concentration. A 0.500 M hydroxide ion concentration gives pOH about 0.301, which corresponds to pH about 13.699. To reach pH 14.000 at 25 C, you would need pOH = 0.000, which would correspond to [OH–] = 1.00 M. Since this solution has 0.500 M OH–, the pH is slightly below 14.
Short conceptual summary
Think of the calculation in two layers. The chemistry layer tells you how many hydroxide ions form from the dissolved base. The math layer converts that concentration into pOH and pH. For Sr(OH)2, the chemistry layer contributes the factor of 2, and the logarithm layer translates concentration into the pH scale.
That means the complete logic is:
- Sr(OH)2 is a strong base.
- It dissociates completely in water.
- Each mole produces 2 moles of OH–.
- 0.250 M base gives 0.500 M hydroxide.
- pOH = -log(0.500) = 0.301.
- pH = 14.00 – 0.301 = 13.699.
Authoritative references for acid-base constants and pH concepts
If you want to verify the scientific framework used here, these resources are reliable starting points:
- National Institute of Standards and Technology (NIST)
- LibreTexts Chemistry, hosted by higher education institutions
- United States Environmental Protection Agency (EPA)
For broad foundational chemistry education from academic and public sector sources, you can also consult university chemistry course pages and federal science references. While specific pH examples may differ in format, the underlying equations and constants are the same.
Final answer
Using complete dissociation of strontium hydroxide at 25 C:
[OH–] = 2(0.250) = 0.500 M
pOH = -log(0.500) = 0.301
pH = 14.00 – 0.301 = 13.699
Therefore, the pH of a 0.250 solution of Sr(OH)2 is 13.699, commonly rounded to 13.70.