Calculate pH for 2.0 × 10-3 M Sr(OH)2
Use this interactive chemistry calculator to find hydroxide concentration, pOH, and pH for strontium hydroxide solutions. The default example below is set to 2.0 × 10-3 M Sr(OH)2, a classic strong base calculation.
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Click Calculate pH to solve for the pH of 2.0 × 10-3 M Sr(OH)2.
How to Calculate pH for 2.0 × 10-3 M Sr(OH)2
If you need to calculate pH for 2.0 × 10-3 M Sr(OH)2, the key idea is that strontium hydroxide is treated as a strong base in standard general chemistry problems. That means it dissociates essentially completely in water, producing one strontium ion and two hydroxide ions per formula unit. Once you know the hydroxide ion concentration, the rest of the calculation is straightforward: determine pOH with the negative logarithm, then use the relationship pH + pOH = 14.00 at 25°C.
This specific problem is common in high school chemistry, AP Chemistry, and first year college chemistry because it tests several important skills at once: interpreting scientific notation, understanding stoichiometry in ionic dissociation, using logarithms correctly, and converting between pOH and pH. Students often make one of two mistakes. First, they forget that Sr(OH)2 contributes two hydroxide ions, not one. Second, they incorrectly use the original base concentration as the hydroxide concentration. Getting the stoichiometry right is what separates the correct answer from a common incorrect result.
Step 1: Write the dissociation equation
Strontium hydroxide is an ionic compound with the formula Sr(OH)2. In water, it dissociates as:
This equation tells you that every 1 mole of Sr(OH)2 produces 2 moles of OH–. That 1:2 ratio is the foundation of the calculation.
Step 2: Convert base concentration into hydroxide concentration
The given concentration is 2.0 × 10-3 M Sr(OH)2. Since each formula unit releases two hydroxide ions:
This is the most important numerical step. The hydroxide concentration is not 2.0 × 10-3 M. It is doubled to 4.0 × 10-3 M because of the subscript 2 in the hydroxide portion of the formula.
Step 3: Calculate pOH
Now apply the definition of pOH:
Substitute the hydroxide concentration:
If your calculator gives something near 2.3979, that is fully consistent. Rounded to two decimal places, the pOH is 2.40.
Step 4: Convert pOH to pH
At 25°C, aqueous solutions follow this relationship:
Therefore:
So the final answer is:
Final Answer for 2.0 × 10-3 M Sr(OH)2
- Given base concentration: 2.0 × 10-3 M Sr(OH)2
- Hydroxide concentration: 4.0 × 10-3 M OH–
- pOH: 2.40
- pH: 11.60
Why Sr(OH)2 Is Treated as a Strong Base
In educational pH problems, metal hydroxides such as NaOH, KOH, and Sr(OH)2 are usually treated as strong bases. A strong base dissociates essentially completely in aqueous solution, which means the stoichiometric amount of hydroxide formed can be used directly in calculations. This makes the math simpler than weak base problems, where you would need an equilibrium expression and a base dissociation constant.
Sr(OH)2 is especially instructive because it is not just a strong base, it is also a base that contributes multiple hydroxide ions. That makes it a good example for learning the difference between the concentration of the compound and the concentration of hydroxide ions generated in solution.
Strong base calculation pattern
- Identify the strong base formula.
- Determine how many OH– ions each unit releases.
- Multiply the formal molarity by that stoichiometric factor.
- Calculate pOH using the hydroxide concentration.
- Convert to pH using 14.00 at 25°C.
Comparison Table: Hydroxide Ion Production by Common Strong Bases
| Base | Dissociation Pattern | OH– Ions per Formula Unit | [OH–] if Base = 2.0 × 10-3 M | Approximate pH at 25°C |
|---|---|---|---|---|
| NaOH | NaOH → Na+ + OH– | 1 | 2.0 × 10-3 M | 11.30 |
| KOH | KOH → K+ + OH– | 1 | 2.0 × 10-3 M | 11.30 |
| Ca(OH)2 | Ca(OH)2 → Ca2+ + 2OH– | 2 | 4.0 × 10-3 M | 11.60 |
| Sr(OH)2 | Sr(OH)2 → Sr2+ + 2OH– | 2 | 4.0 × 10-3 M | 11.60 |
| Ba(OH)2 | Ba(OH)2 → Ba2+ + 2OH– | 2 | 4.0 × 10-3 M | 11.60 |
This table highlights a practical rule: compounds with two hydroxide groups give twice the hydroxide concentration of compounds with one hydroxide group, assuming the same formal molarity and complete dissociation. That difference changes the pOH and therefore the pH.
Common Student Mistakes When Solving This Problem
- Forgetting the coefficient of 2 for hydroxide: Sr(OH)2 does not produce one hydroxide ion. It produces two.
- Taking the negative log of the base concentration instead of [OH–]: pOH must be based on the hydroxide concentration.
- Mixing up pH and pOH: A base should have a pH above 7 at 25°C.
- Ignoring significant figures: With a concentration of 2.0 × 10-3 M, reporting pH as 11.60 is usually appropriate in textbook settings.
- Using weak base methods: You do not need an ICE table or Kb for this standard strong base problem.
Detailed Numerical Breakdown
Let us look at the arithmetic in a more explicit way. The original concentration can be written in decimal form as 0.0020 M. Multiplying by 2 gives:
Then:
And:
Rounding appropriately:
Comparison Table: pH Trends Across Related Concentrations of Sr(OH)2
| Sr(OH)2 Concentration (M) | [OH–] Produced (M) | pOH | pH at 25°C | Interpretation |
|---|---|---|---|---|
| 2.0 × 10-4 | 4.0 × 10-4 | 3.40 | 10.60 | Basic, but less concentrated |
| 2.0 × 10-3 | 4.0 × 10-3 | 2.40 | 11.60 | Standard target problem |
| 2.0 × 10-2 | 4.0 × 10-2 | 1.40 | 12.60 | Much stronger basicity |
| 1.0 × 10-1 | 2.0 × 10-1 | 0.70 | 13.30 | Highly basic solution |
Notice the logarithmic behavior: a tenfold increase in concentration changes pOH by about 1 unit, which changes pH by about 1 unit in the opposite direction. This is why pH scales can feel non intuitive at first. The values do not increase linearly with concentration.
When This Method Works Best
The direct stoichiometric method used here works best in standard dilute to moderately concentrated classroom solutions where the strong base assumption is valid and the temperature is taken as 25°C. In more advanced chemical analysis, real solutions may involve activity corrections, non ideality, limited solubility, or temperature dependent values of Kw. Those effects are usually ignored in introductory chemistry unless a problem explicitly asks for them.
For this reason, if your textbook, teacher, or exam presents “calculate pH for 2.0 × 10-3 M Sr(OH)2,” the intended solution is almost always:
- Double the concentration to get [OH–].
- Calculate pOH.
- Subtract from 14.00 to get pH.
Authoritative Chemistry References
For additional background on pH, hydroxide concentration, and acid-base chemistry, review these authoritative educational and scientific sources:
- U.S. Environmental Protection Agency: pH Basics
- Chemistry LibreTexts Educational Resource
- U.S. Geological Survey: pH and Water
Quick Recap
To calculate pH for 2.0 × 10-3 M Sr(OH)2, remember that strontium hydroxide releases two hydroxide ions per formula unit. That doubles the hydroxide concentration to 4.0 × 10-3 M. Taking the negative logarithm gives a pOH of 2.40, and subtracting from 14.00 gives a pH of 11.60. If you master this pattern, you will be able to solve nearly any strong base pH problem involving hydroxides of Group 1 and many Group 2 metals.