1.54 × 10-4 M Sr(OH)2: Calculate the pH
Use this premium calculator to find the pH, pOH, and hydroxide concentration for strontium hydroxide solutions. The default example is 1.54 × 10-4 M Sr(OH)2, a common chemistry homework format. Since Sr(OH)2 is treated as a strong base in introductory chemistry, each mole produces 2 moles of OH–.
Results
Enter values and click Calculate pH to see the worked answer.
How to calculate the pH of 1.54 × 10-4 M Sr(OH)2
If your chemistry problem says 1.54 10 4 m sr oh 2 calculate the ph, it almost always means 1.54 × 10-4 M Sr(OH)2. The task is to determine the pH of a strontium hydroxide solution. This is a standard general chemistry question that tests your understanding of strong bases, ion dissociation, hydroxide concentration, pOH, and finally pH.
The key idea is that strontium hydroxide, Sr(OH)2, dissociates to produce two hydroxide ions for every one formula unit dissolved in water:
Sr(OH)2 → Sr2+ + 2OH–
Because of this 1:2 relationship, the hydroxide ion concentration is not equal to the original molarity of Sr(OH)2. Instead, it is twice the formal concentration, assuming complete dissociation under introductory chemistry conditions. That one step is where many students lose points, so it is worth emphasizing from the start.
Step-by-step solution for 1.54 × 10-4 M Sr(OH)2
- Start with the given base concentration: [Sr(OH)2] = 1.54 × 10-4 M.
- Use the dissociation stoichiometry. Since each Sr(OH)2 yields 2 OH–, calculate: [OH–] = 2 × 1.54 × 10-4 = 3.08 × 10-4 M.
- Find pOH using the definition: pOH = -log[OH–].
- Substitute the hydroxide concentration: pOH = -log(3.08 × 10-4) ≈ 3.511.
- At 25 degrees C, use the standard relationship: pH + pOH = 14.00.
- Therefore: pH = 14.00 – 3.511 = 10.489.
Final answer: pH ≈ 10.49 for a 1.54 × 10-4 M Sr(OH)2 solution at 25 degrees C.
Why Sr(OH)2 changes the pH so efficiently
Sr(OH)2 is a metal hydroxide and is usually treated as a strong base in general chemistry. Strong bases are assumed to dissociate essentially completely in water, which means the amount of hydroxide ion generated can be calculated directly from the formula. In contrast, weak bases require an equilibrium expression involving Kb, ICE tables, and approximation checks.
In this case, complete dissociation makes the problem straightforward. The only twist is the subscript 2 in the formula. Since there are two hydroxide groups attached to strontium, the hydroxide ion concentration doubles relative to the formal molarity. If you forget that factor, you would incorrectly use 1.54 × 10-4 M for [OH–] and get the wrong pOH and pH.
Common student mistakes
- Using [OH–] = 1.54 × 10-4 M instead of 3.08 × 10-4 M.
- Calculating pH directly from the base concentration without finding pOH first.
- Forgetting that pH + pOH = 14.00 only applies exactly at 25 degrees C in standard classroom treatment.
- Entering scientific notation incorrectly into a calculator.
- Mixing up strong base dissociation with weak base equilibrium methods.
Scientific notation and calculator entry tips
The phrase written by students often looks compressed, such as 1.54 10 4 m sr oh 2 calculate the ph. In a chemistry textbook or formal notation, that should be read as 1.54 × 10-4 M Sr(OH)2. On a scientific calculator, you might enter this as 1.54 EXP -4 or 1.54E-4, depending on the model.
Scientific notation matters because concentrations in chemistry are often very small. If you entered 1.54 × 104 M instead of 1.54 × 10-4 M, you would be off by a factor of 100,000,000. That would lead to a physically unrealistic answer and a completely different pH.
Worked comparison of common strong bases
| Base | Dissociation Pattern | OH– Released per Formula Unit | If Base Concentration = 1.54 × 10-4 M, Then [OH–] | Approximate pH at 25 degrees C |
|---|---|---|---|---|
| NaOH | NaOH → Na+ + OH– | 1 | 1.54 × 10-4 M | 10.19 |
| KOH | KOH → K+ + OH– | 1 | 1.54 × 10-4 M | 10.19 |
| Sr(OH)2 | Sr(OH)2 → Sr2+ + 2OH– | 2 | 3.08 × 10-4 M | 10.49 |
| Ba(OH)2 | Ba(OH)2 → Ba2+ + 2OH– | 2 | 3.08 × 10-4 M | 10.49 |
This comparison shows why stoichiometry matters. Bases with one hydroxide ion per formula unit generate less OH– at the same formal molarity than bases with two hydroxide ions. Sr(OH)2 and Ba(OH)2 therefore produce a higher pH than NaOH or KOH when all are present at the same concentration.
Definitions you should know
- Molarity (M): moles of solute per liter of solution.
- [OH–]: hydroxide ion concentration in mol/L.
- pOH: negative base-10 logarithm of hydroxide concentration.
- pH: negative base-10 logarithm of hydrogen ion concentration.
- At 25 degrees C: pH + pOH = 14.00.
- Strong base: a base that dissociates essentially completely in water in general chemistry treatment.
Numerical checkpoints for this problem
| Quantity | Value | Meaning |
|---|---|---|
| Given concentration | 1.54 × 10-4 M | Formal molarity of Sr(OH)2 |
| Hydroxide multiplier | 2 | Two OH– ions per Sr(OH)2 |
| [OH–] | 3.08 × 10-4 M | Actual hydroxide ion concentration |
| pOH | 3.511 | Calculated from -log[OH–] |
| pH | 10.489 | Calculated from 14.00 – pOH |
Is autoionization of water important here?
At very low concentrations of strong acids or bases, the autoionization of water can become important. Pure water at 25 degrees C has [H+] = [OH–] = 1.0 × 10-7 M. In this problem, the calculated hydroxide concentration from Sr(OH)2 is 3.08 × 10-4 M, which is much larger than 1.0 × 10-7 M. That means the base contribution overwhelmingly dominates, and the standard strong-base approach is entirely appropriate.
In other words, for 1.54 × 10-4 M Sr(OH)2, you do not need a more advanced correction. The simple method taught in general chemistry gives an accurate educational answer: pH ≈ 10.49.
How this connects to broader chemistry topics
Problems like this are not just about getting a number. They reinforce several core chemistry skills:
- Reading and rewriting scientific notation correctly.
- Recognizing whether a compound is a strong acid, strong base, weak acid, or weak base.
- Using dissociation stoichiometry to convert formula concentration into ion concentration.
- Applying logarithms properly for pH and pOH calculations.
- Checking whether your answer is chemically reasonable.
A reasonable answer check is simple here: because Sr(OH)2 is a base, the pH must be above 7. Since the concentration is relatively low, the pH should be basic but not extreme. A result around 10.5 fits that expectation well.
Authority references for pH and water chemistry
For foundational chemistry and water quality information, consult authoritative educational and government resources:
- USGS: pH and Water
- Chemistry LibreTexts educational chemistry resources
- U.S. EPA: pH overview and environmental relevance
Although classroom pH calculations are often idealized, these sources help connect the math to real-world chemistry in natural waters, treatment systems, and laboratory measurements.
Final answer summary
To calculate the pH of 1.54 × 10-4 M Sr(OH)2, multiply the base concentration by 2 because each formula unit releases two hydroxide ions:
[OH–] = 3.08 × 10-4 M
pOH = 3.511
pH = 10.489 ≈ 10.49
That is the correct general chemistry result at 25 degrees C. If you are solving this for homework, quiz review, or exam practice, remember the main idea: always account for the number of OH– ions produced by the base. For Sr(OH)2, that factor is 2, and it changes the final pH noticeably.