Calculate pH for 1.5 × 10-3 M Sr(OH)2
Use this interactive calculator to find hydroxide concentration, pOH, and pH for strontium hydroxide solutions. The default setup solves the classic chemistry problem for 1.5 × 10-3 M Sr(OH)2, a strong base that dissociates into one Sr2+ ion and two OH– ions.
Calculator
Core Method
[OH-] = n × C
pOH = -log10([OH-])
pH = 14 - pOH
For Sr(OH)2, n = 2
Visual Breakdown
The chart compares the original molarity of Sr(OH)2, the resulting hydroxide ion concentration, the pOH, and the final pH. This makes it easier to see how a relatively small molar concentration can still produce a strongly basic solution because each formula unit contributes two hydroxide ions.
How to calculate pH for 1.5 × 10-3 M Sr(OH)2
To calculate pH for 1.5 × 10-3 M Sr(OH)2, you must recognize that strontium hydroxide is a strong base. In introductory and general chemistry, it is treated as fully dissociated in water. That means each mole of Sr(OH)2 separates into one Sr2+ ion and two OH– ions. This point matters because pH is not found directly from the concentration of the compound itself. Instead, for a base, you first calculate the hydroxide ion concentration, then determine pOH, and finally convert pOH into pH.
The given concentration is 1.5 × 10-3 M Sr(OH)2. Because one unit of Sr(OH)2 produces two hydroxide ions, the hydroxide ion concentration is:
[OH–] = 2 × 1.5 × 10-3 = 3.0 × 10-3 M
Next, use the definition of pOH:
pOH = -log10(3.0 × 10-3) ≈ 2.523
At 25 degrees C, pH and pOH are related by the equation:
pH + pOH = 14.00
So the pH becomes:
pH = 14.00 – 2.523 = 11.477
This means the correct pH is approximately 11.48. That value indicates a strongly basic solution. Even though 1.5 × 10-3 M may appear numerically small, strong bases can still generate high pH values because the logarithmic pH scale compresses concentration changes into relatively compact numbers.
Step by step solution
- Identify Sr(OH)2 as a strong base.
- Write its dissociation: Sr(OH)2 → Sr2+ + 2OH–.
- Multiply the base concentration by 2 to get hydroxide concentration.
- Use pOH = -log[OH–].
- Use pH = 14 – pOH at 25 degrees C.
- Round properly based on your chemistry course precision rules.
Why the factor of 2 matters for Sr(OH)2
One of the most common mistakes students make is forgetting the stoichiometric coefficient of hydroxide. Sodium hydroxide, NaOH, releases one hydroxide ion per formula unit, so its hydroxide concentration matches its molarity. Strontium hydroxide does not work that way. It releases two hydroxide ions per mole. If you ignore that and incorrectly set [OH–] equal to 1.5 × 10-3 M, you would calculate:
- Incorrect pOH = -log(1.5 × 10-3) ≈ 2.824
- Incorrect pH ≈ 11.176
That wrong result is lower than the correct answer by roughly 0.30 pH units. Since pH is logarithmic, a 0.30 difference is not trivial. In fact, a difference of 0.30 corresponds to roughly a factor of 2 in hydrogen or hydroxide concentration. This is exactly what you would expect when the missing stoichiometric factor is 2.
| Compound | Formula | OH- ions released per mole | If concentration = 1.5 × 10^-3 M, [OH-] would be |
|---|---|---|---|
| Sodium hydroxide | NaOH | 1 | 1.5 × 10^-3 M |
| Strontium hydroxide | Sr(OH)2 | 2 | 3.0 × 10^-3 M |
| Barium hydroxide | Ba(OH)2 | 2 | 3.0 × 10^-3 M |
| Calcium hydroxide | Ca(OH)2 | 2 | 3.0 × 10^-3 M |
Worked chemistry interpretation of the answer
A final pH of approximately 11.48 places the solution clearly in the basic region. Neutral water at 25 degrees C has a pH of 7.00, so this solution is 4.48 pH units above neutral. Because each pH unit corresponds to a tenfold change in hydrogen ion concentration, that is a substantial difference. Specifically, compared with neutral water, the hydrogen ion concentration in this solution is far lower, while the hydroxide ion concentration is much higher.
Using the relationship pH + pOH = 14.00, a pOH of 2.523 tells you the hydroxide concentration is around 10-2.523 M, which matches 3.0 × 10-3 M. This internal consistency is important in chemistry. Good calculation habits include checking that your logarithm result agrees with the original concentration scale. If your pOH were around 5 or 6, for example, that would not be compatible with an OH– concentration in the 10-3 range.
Quick reasonableness check
- A concentration in the low 10-3 M range should produce a pOH near 3.
- If pOH is near 3, then pH should be near 11.
- Because [OH–] is 3.0 × 10-3, a pOH slightly less than 3 makes sense.
- Therefore a pH slightly greater than 11, specifically 11.48, is reasonable.
Comparison table: correct vs common mistakes
| Method | Computed [OH-] | Computed pOH | Computed pH | Issue |
|---|---|---|---|---|
| Correct method for Sr(OH)2 | 3.0 × 10^-3 M | 2.523 | 11.477 | Uses 2 OH- per formula unit |
| Forgetting stoichiometric factor | 1.5 × 10^-3 M | 2.824 | 11.176 | Too low by about 0.301 pH units |
| Using pH = -log[OH-] | 3.0 × 10^-3 M | Not used | 2.523 | Confuses pH with pOH |
Real scientific context and reference values
When studying pH, it helps to anchor calculations to accepted scientific standards. At 25 degrees C, the ion-product constant for water is commonly taken as Kw = 1.0 × 10-14, which leads to the standard relationship pH + pOH = 14.00. Educational resources from major chemistry departments and government sources consistently use this framework in introductory aqueous equilibrium problems. A pH around 11.5 is therefore perfectly consistent with a dilute but fully dissociated strong base.
For perspective, here are useful benchmark values often encountered in general chemistry:
- Pure water at 25 degrees C: pH 7.00
- 1.0 × 10-3 M NaOH: pOH 3.00, pH 11.00
- 1.5 × 10-3 M NaOH: pOH 2.824, pH 11.176
- 1.5 × 10-3 M Sr(OH)2: pOH 2.523, pH 11.477
This comparison shows why Sr(OH)2 gives a higher pH than an equal molarity solution of NaOH. The difference comes entirely from the larger number of hydroxide ions released per dissolved formula unit.
Common student questions about this calculation
Do I need an ICE table?
No, not for the standard form of this problem. Since Sr(OH)2 is treated as a strong base in general chemistry, it dissociates essentially completely. An ICE table is more useful for weak acids, weak bases, hydrolysis problems, or equilibrium situations where dissociation is partial.
Why do we calculate pOH first instead of pH directly?
Because the species you know immediately from a strong base is OH–, not H+. The pH scale is tied to hydrogen ion concentration, while pOH is tied to hydroxide ion concentration. Once you know pOH, converting to pH is straightforward using pH + pOH = 14.
What if the temperature is not 25 degrees C?
The relation pH + pOH = 14.00 is exact only for 25 degrees C when Kw is 1.0 × 10-14. At other temperatures, Kw changes, so the sum is not exactly 14. However, for standard textbook problems like this one, 25 degrees C is assumed unless stated otherwise.
Does solubility ever matter?
In some advanced or high-concentration situations, yes. But for a typical classroom problem giving a direct molarity for dissolved Sr(OH)2, you use that concentration as the aqueous concentration and calculate pH from complete dissociation.
Best practices for solving base pH problems accurately
- Classify the substance first: strong acid, weak acid, strong base, or weak base.
- Write the dissociation equation so you can see stoichiometric ratios clearly.
- Convert compound concentration into ion concentration before taking a logarithm.
- Use pOH for bases and pH for acids unless a direct conversion is easier.
- Check whether the final pH is above 7 for a base and below 7 for an acid.
- Verify that your answer matches the order of magnitude of the concentration.
Authoritative references for pH, pOH, and aqueous chemistry
U.S. Environmental Protection Agency: pH overview
University-level chemistry reference library
U.S. Geological Survey: pH and water
Final answer for 1.5 × 10-3 M Sr(OH)2
If you need the concise exam-style result, here it is:
- Given concentration of Sr(OH)2 = 1.5 × 10-3 M
- Hydroxide concentration = 2 × 1.5 × 10-3 = 3.0 × 10-3 M
- pOH = -log(3.0 × 10-3) = 2.523
- pH = 14.000 – 2.523 = 11.477
Therefore, the pH of 1.5 × 10-3 M Sr(OH)2 is approximately 11.48.