16 Calculate the pH of 0.0075 M Sr(OH)2
Use this premium calculator to determine hydroxide concentration, pOH, and pH for a strontium hydroxide solution. The tool assumes ideal strong-base dissociation unless you choose a different ionization model. A chart updates instantly to visualize the chemistry behind the answer.
pH Calculator
Enter the concentration and solution assumptions. For the standard problem, use 0.0075 M Sr(OH)2 at 25°C with complete dissociation.
Calculated Results
This is the expected answer for a 0.0075 M Sr(OH)2 solution at 25°C, assuming complete dissociation.
How to calculate the pH of 0.0075 M Sr(OH)2
If your chemistry problem asks, calculate the pH of 0.0075 M Sr(OH)2, the core idea is that strontium hydroxide is treated as a strong base in introductory and general chemistry. That means it dissociates essentially completely in water and releases hydroxide ions according to the formula:
Sr(OH)2 → Sr2+ + 2OH–
The crucial stoichiometric detail is the coefficient in front of hydroxide. One mole of Sr(OH)2 produces two moles of OH–. Because the solution concentration is 0.0075 M, the hydroxide concentration becomes:
[OH–] = 2 × 0.0075 = 0.0150 M
Once you know hydroxide concentration, find pOH using the negative base-10 logarithm:
pOH = -log(0.0150) = 1.8239
At 25°C, water follows the familiar relationship:
pH + pOH = 14.00
So the pH is:
pH = 14.00 – 1.8239 = 12.1761
Rounded to two decimal places, the answer is 12.18. Rounded to three decimal places, the answer is 12.176. This is a strongly basic solution, well above neutral pH 7.
Step-by-step solution
- Identify Sr(OH)2 as a strong base.
- Write its dissociation: Sr(OH)2 → Sr2+ + 2OH–.
- Multiply the base molarity by 2 because each mole gives two moles of hydroxide.
- Calculate hydroxide concentration: [OH–] = 0.0150 M.
- Use pOH = -log[OH–] to get pOH = 1.8239.
- Use pH = 14.00 – pOH to get pH = 12.1761 at 25°C.
Why Sr(OH)2 contributes two hydroxide ions
The formula Sr(OH)2 contains one strontium ion and two hydroxide groups. When the compound dissolves, the ionic components separate. Unlike a base such as NaOH, which contributes only one hydroxide per formula unit, strontium hydroxide contributes two. This is why students often make a mistake if they set [OH–] equal to 0.0075 M instead of 0.0150 M. That single oversight changes the pOH and the final pH significantly.
A good chemistry habit is to always inspect the subscript on hydroxide in any metal hydroxide. For compounds such as Mg(OH)2, Ca(OH)2, Sr(OH)2, and Ba(OH)2, the subscript 2 means the hydroxide concentration is twice the base molarity if the base fully dissociates.
Common student mistakes
- Using 0.0075 M directly as [OH–] instead of doubling it.
- Calculating pH directly from the base concentration without first finding pOH.
- Forgetting that pH + pOH = 14 only applies exactly at 25°C.
- Entering the logarithm incorrectly on a calculator.
- Confusing strong bases with weak bases, which would require an equilibrium approach.
Quick check for reasonableness
Is a pH of about 12.18 reasonable? Yes. A hydroxide concentration of 0.0150 M is far greater than the 1.0 × 10-7 M hydroxide concentration associated with neutral water at 25°C. Because the solution has much more OH– than pure water, the pOH must be fairly small, and the pH must be well above 7. A value near 12 fits that expectation.
| Sr(OH)2 Molarity (M) | OH- Stoichiometric Factor | [OH-] (M) | pOH at 25°C | pH at 25°C |
|---|---|---|---|---|
| 0.0010 | 2 | 0.0020 | 2.6990 | 11.3010 |
| 0.0050 | 2 | 0.0100 | 2.0000 | 12.0000 |
| 0.0075 | 2 | 0.0150 | 1.8239 | 12.1761 |
| 0.0100 | 2 | 0.0200 | 1.6990 | 12.3010 |
| 0.0500 | 2 | 0.1000 | 1.0000 | 13.0000 |
Interpreting the answer chemically
A pH of 12.1761 indicates a strongly alkaline solution. In practical terms, this means the solution has a substantial excess of hydroxide ions compared with hydronium ions. Since pH is logarithmic, even a change of 1 pH unit corresponds to a tenfold change in hydrogen ion activity. Therefore, a pH around 12.18 is not just mildly basic; it is strongly basic.
In many educational settings, problems like this are designed to test several ideas at once: identifying a strong base, understanding dissociation, using stoichiometric coefficients correctly, and applying logarithms. If you can solve this problem confidently, you are also prepared for many similar pH and pOH calculations involving group 1 hydroxides and group 2 hydroxides.
Comparison with other common bases
It helps to compare Sr(OH)2 with more familiar bases:
- NaOH: one hydroxide per formula unit, so [OH–] = base molarity.
- KOH: one hydroxide per formula unit, same stoichiometric rule as NaOH.
- Ca(OH)2: two hydroxides per formula unit, same stoichiometric rule as Sr(OH)2.
- Ba(OH)2: two hydroxides per formula unit, also similar in textbook pH calculations.
That means if NaOH and Sr(OH)2 have the same molarity, the Sr(OH)2 solution contributes twice as much hydroxide, giving a lower pOH and a higher pH.
| Substance | Molarity (M) | OH- Released per Formula Unit | Effective [OH-] (M) | Calculated pH at 25°C |
|---|---|---|---|---|
| NaOH | 0.0075 | 1 | 0.0075 | 11.8751 |
| KOH | 0.0075 | 1 | 0.0075 | 11.8751 |
| Ca(OH)2 | 0.0075 | 2 | 0.0150 | 12.1761 |
| Sr(OH)2 | 0.0075 | 2 | 0.0150 | 12.1761 |
| Ba(OH)2 | 0.0075 | 2 | 0.0150 | 12.1761 |
Temperature and pKw considerations
In standard introductory chemistry, pH + pOH = 14.00 is usually used because problems are assumed to occur at 25°C. In more advanced chemistry, pKw changes with temperature, so that sum is not always exactly 14. That is why this calculator includes a temperature selector. For nearly all textbook exercises written exactly like “calculate the pH of 0.0075 M Sr(OH)2,” the expected convention is 25°C unless otherwise stated.
If you were working in a laboratory or preparing a detailed report, you would also consider ionic strength, activity effects, and more accurate thermodynamic relationships. But for normal pH homework, complete dissociation and pKw = 14.00 are the accepted assumptions.
How this type of question appears on exams
Teachers often vary this problem slightly to check whether you understand the method rather than memorizing one answer. You might see any of the following:
- Find the pOH of 0.020 M Sr(OH)2.
- Find the hydroxide concentration in 0.0030 M Ba(OH)2.
- Compare the pH of 0.010 M NaOH and 0.010 M Ca(OH)2.
- Determine whether a metal hydroxide solution is acidic, neutral, or basic.
Every one of these depends on the same conceptual sequence: write the dissociation, determine [OH–], calculate pOH, then calculate pH.
Memorize this fast method
For a strong base with formula M(OH)2, you can move quickly:
- Double the molarity.
- Take the negative log to get pOH.
- Subtract from 14.00 to get pH at 25°C.
Applied here:
- Double 0.0075 to get 0.0150
- pOH = -log(0.0150) = 1.8239
- pH = 14.00 – 1.8239 = 12.1761
Authoritative references for pH and hydroxide calculations
For readers who want reliable background from scientific and educational institutions, these sources are useful:
Final answer
For the problem calculate the pH of 0.0075 M Sr(OH)2, assuming complete dissociation at 25°C:
- [OH–] = 0.0150 M
- pOH = 1.8239
- pH = 12.1761
The best concise exam-ready statement is: The pH of 0.0075 M Sr(OH)2 is 12.18.