Calculate pH for 1.1 × 10-3 M Sr(OH)2
Use this premium calculator to find hydroxide concentration, pOH, and pH for strontium hydroxide solutions. The default example is the exact problem: calculate pH for 1.1 × 10-3 M Sr(OH)2 at 25°C, assuming complete dissociation.
Expert Guide: How to Calculate pH for 1.1 × 10-3 M Sr(OH)2
To calculate pH for 1.1 × 10-3 M Sr(OH)2, you treat strontium hydroxide as a strong base under standard general chemistry conditions. That means it dissociates essentially completely in water:
The key insight is that each mole of Sr(OH)2 produces two moles of hydroxide ions. So if the strontium hydroxide concentration is 1.1 × 10-3 M, the hydroxide concentration is:
Next, compute pOH using the base-10 logarithm:
Finally, at 25°C you use the relation:
So the final answer is pH ≈ 11.34. This is the standard textbook answer for the problem. The rest of this guide explains why that is correct, what assumptions are involved, and how this value compares with other strong bases and concentrations.
Step-by-step method for this exact problem
- Write the dissociation equation for Sr(OH)2.
- Identify the stoichiometric ratio of hydroxide ions to formula units: 2 OH– per 1 Sr(OH)2.
- Multiply the given molarity by 2 to get hydroxide concentration.
- Use pOH = -log[OH–].
- Use pH = 14 – pOH at 25°C.
This method works because strontium hydroxide is classified as a strong base in general chemistry treatment. For many instructional problems, complete dissociation is assumed unless the course specifically introduces nonideal effects or activity corrections.
Why Sr(OH)2 contributes twice as much OH–
Students often make one critical mistake in this problem: they forget the coefficient of hydroxide. Sr(OH)2 contains two hydroxide groups, so the concentration of OH– is not the same as the concentration of Sr(OH)2. It is double. If you used 1.1 × 10-3 M directly as [OH–], you would calculate the wrong pOH and the wrong pH.
- Given base concentration: 1.1 × 10-3 M
- Hydroxide multiplier for Sr(OH)2: 2
- Actual hydroxide concentration: 2.2 × 10-3 M
- Correct pOH: about 2.66
- Correct pH: about 11.34
Worked logarithm check
If you want to see the logarithm split into parts, here is the same calculation in expanded form:
Rounded to two decimal places, the pH is 11.34. Depending on your instructor or textbook, you may report 11.34 or 11.342. The right number of significant figures is usually guided by the concentration data and course policy.
Common mistakes when calculating pH for 1.1 × 10-3 M Sr(OH)2
- Forgetting the 2 in Sr(OH)2: This is the most frequent error.
- Taking log of the base concentration instead of hydroxide concentration: You must use [OH–], not [Sr(OH)2].
- Confusing pH and pOH: For bases, you usually calculate pOH first.
- Using pH = -log[OH–]: That expression gives pOH, not pH.
- Ignoring temperature assumptions: The relation pH + pOH = 14 is exact only at 25°C in introductory treatment.
Comparison table: pH values for similar strong bases at the same formal concentration
The table below shows how stoichiometry changes hydroxide concentration and pH, even when the formal molarity is the same. These values assume complete dissociation at 25°C.
| Base | Formal concentration (M) | OH– released per formula unit | [OH–] (M) | pOH | pH |
|---|---|---|---|---|---|
| NaOH | 1.1 × 10-3 | 1 | 1.1 × 10-3 | 2.96 | 11.04 |
| KOH | 1.1 × 10-3 | 1 | 1.1 × 10-3 | 2.96 | 11.04 |
| Ca(OH)2 | 1.1 × 10-3 | 2 | 2.2 × 10-3 | 2.66 | 11.34 |
| Sr(OH)2 | 1.1 × 10-3 | 2 | 2.2 × 10-3 | 2.66 | 11.34 |
| Ba(OH)2 | 1.1 × 10-3 | 2 | 2.2 × 10-3 | 2.66 | 11.34 |
This makes the stoichiometric lesson clear: the pH for a strong dibasic hydroxide is higher than the pH of a strong monobasic hydroxide at the same formula-unit concentration, because the hydroxide concentration is larger by a factor of two.
How concentration changes pH for Sr(OH)2
Because pOH depends on the logarithm of hydroxide concentration, pH does not rise linearly with concentration. A tenfold increase in concentration changes pOH by 1 unit and therefore changes pH by 1 unit at 25°C. The table below shows example values for Sr(OH)2.
| Sr(OH)2 concentration (M) | [OH–] (M) | pOH | pH at 25°C |
|---|---|---|---|
| 1.0 × 10-4 | 2.0 × 10-4 | 3.70 | 10.30 |
| 5.0 × 10-4 | 1.0 × 10-3 | 3.00 | 11.00 |
| 1.1 × 10-3 | 2.2 × 10-3 | 2.66 | 11.34 |
| 1.0 × 10-2 | 2.0 × 10-2 | 1.70 | 12.30 |
| 1.0 × 10-1 | 2.0 × 10-1 | 0.70 | 13.30 |
What assumptions are built into the answer?
When a chemistry class asks you to calculate pH for 1.1 × 10-3 M Sr(OH)2, the expected answer nearly always uses several standard assumptions:
- Complete dissociation: Sr(OH)2 is treated as a strong electrolyte in dilute aqueous solution.
- 25°C temperature: This lets you use pH + pOH = 14.00.
- Ideal solution behavior: Activities are approximated by concentrations.
- Negligible water autoionization contribution: The OH– from the base is far greater than 1.0 × 10-7 M, so water’s contribution is insignificant.
These assumptions are fully appropriate for most homework, quiz, and exam settings in introductory chemistry. In more advanced analytical chemistry, physical chemistry, or environmental chemistry, you may need to consider ionic strength, activity coefficients, and temperature-adjusted pKw values. However, those refinements do not usually change the main educational point of this specific exercise.
Why pH + pOH = 14 is a temperature-specific rule
The familiar equation pH + pOH = 14 comes from the ionic product of water. At 25°C, the value is approximately 1.0 × 10-14, so pKw is 14.00. Outside 25°C, pKw changes, which means pH + pOH also changes. If your instructor specifies a different temperature or gives a custom pKw, you should use that value instead of 14.
If no temperature is given, 25°C is the standard assumption. That is exactly why the calculator above includes a custom pKw option, but defaults to 14.00.
Where this calculation matters in real chemistry
Strong-base pH calculations are not just classroom drills. Similar ideas are used in industrial, environmental, and laboratory work. Alkalinity, neutralization, precipitation chemistry, water treatment, and reagent preparation all rely on understanding how hydroxide concentration maps to pOH and pH.
Authoritative educational and scientific references that support these core acid-base principles include:
- Chemistry LibreTexts educational resource
- U.S. Environmental Protection Agency
- U.S. Geological Survey
- National Institute of Standards and Technology
For example, the U.S. Geological Survey explains the practical meaning of pH in water systems, while the EPA discusses water chemistry and quality concepts that depend on acid-base measurements. NIST provides standards and scientific reference frameworks that underpin accurate chemical measurement.
Quick recap of the final answer
If you need the shortest possible solution for a worksheet, this is it:
Final answer: pH = 11.34
Bottom line
To calculate pH for 1.1 × 10-3 M Sr(OH)2, remember that strontium hydroxide releases two hydroxide ions per formula unit. That doubles the hydroxide concentration to 2.2 × 10-3 M. From there, the logarithm gives pOH ≈ 2.66, and at 25°C the pH is 11.34. If you remember the dissociation stoichiometry, this type of problem becomes fast and reliable.