16 calculate the pH of 0.0075 M Sr(OH)2
Use this premium chemistry calculator to find the hydroxide concentration, pOH, and pH of a strontium hydroxide solution. The default values are set to the exact problem: 0.0075 M Sr(OH)2 at 25 C.
- Assumes complete dissociation for Sr(OH)2 as a strong base.
- Uses pOH = -log10[OH-] and pH = 14 – pOH at 25 C.
- Supports quick recalculation for any concentration and hydroxide stoichiometry.
How to calculate the pH of 0.0075 M Sr(OH)2
The question “calculate the pH of 0.0075 M Sr(OH)2” is a classic general chemistry problem about strong bases, stoichiometric dissociation, and the relationship between hydroxide concentration, pOH, and pH. The correct answer at 25 C is approximately 12.18. To understand why, you need to move carefully through three ideas: first, recognize that strontium hydroxide is treated as a strong base in introductory chemistry; second, account for the fact that each formula unit contributes two hydroxide ions; and third, convert hydroxide concentration into pOH and then pH.
Sr(OH)2 is strontium hydroxide. In water, it dissociates according to the equation:
Sr(OH)2 → Sr2+ + 2OH-
This means every 1 mole of dissolved Sr(OH)2 produces 2 moles of OH-. If the original solution concentration is 0.0075 M, then the hydroxide concentration is not 0.0075 M. Instead, it is doubled:
[OH-] = 2 × 0.0075 = 0.015 M
Now apply the pOH formula:
pOH = -log10(0.015) ≈ 1.82
Finally, at 25 C:
pH = 14.00 – 1.82 = 12.18
That is the full calculation in compact form, but there is a lot of chemistry packed into those steps. The sections below explain the reasoning in detail, show common mistakes, compare Sr(OH)2 with other hydroxide bases, and provide reference data that can help you solve similar pH questions much faster.
Step by step solution for 0.0075 M Sr(OH)2
- Identify the compound as a strong base. Strontium hydroxide is generally treated as completely dissociated in standard chemistry pH problems.
- Write the dissociation equation. Sr(OH)2 → Sr2+ + 2OH-.
- Determine hydroxide stoichiometry. One formula unit releases two OH- ions.
- Compute hydroxide concentration. [OH-] = 2 × 0.0075 M = 0.015 M.
- Calculate pOH. pOH = -log10(0.015) = 1.8239.
- Calculate pH. pH = 14.0000 – 1.8239 = 12.1761.
- Round appropriately. To two decimal places, pH ≈ 12.18.
Why the factor of 2 matters
The most common error on this question is forgetting the stoichiometric multiplier from the chemical formula. Students often look at 0.0075 M and immediately compute pOH as if [OH-] were also 0.0075 M. That gives a pOH of about 2.12 and a pH of about 11.88, which is too low. The reason is simple: strontium hydroxide contains two hydroxide groups, not one. Since hydroxide is what determines basicity in this problem, you must count both OH- ions released per dissolved unit of Sr(OH)2.
This same idea applies to other metal hydroxides. Sodium hydroxide, NaOH, contributes one OH- per mole. Calcium hydroxide, Ca(OH)2, contributes two. Aluminum hydroxide, Al(OH)3, would contribute three in a pure stoichiometric sense, although real acid-base behavior can be more complex depending on solubility and amphoterism. For a straightforward strong base problem, always inspect the formula before starting the logarithms.
Key formulas used in this calculator
- Hydroxide concentration: [OH-] = base molarity × number of OH- groups
- pOH: pOH = -log10[OH-]
- pH at 25 C: pH = 14.00 – pOH
- Water relation: pH + pOH = 14.00
These formulas rely on the standard 25 C value of pKw = 14.00. In more advanced chemistry, pKw changes slightly with temperature, so the exact pH plus pOH sum is not always 14. However, unless your instructor or problem statement specifies another temperature framework, using 14.00 is the accepted method for classroom calculations.
Worked numeric check
Let us verify the answer with a clean numerical walkthrough. Start with the concentration of strontium hydroxide:
C = 0.0075 M
Because Sr(OH)2 produces 2 hydroxide ions:
[OH-] = 2C = 2(0.0075) = 0.015 M
Now take the common logarithm:
log10(0.015) = -1.8239
Therefore:
pOH = 1.8239
And:
pH = 14.0000 – 1.8239 = 12.1761
Rounded answer:
pH = 12.18
Comparison table: pH of common hydroxide bases at 0.0075 M
The table below shows how stoichiometry changes the result even when the initial base molarity is the same. These are calculated values at 25 C, assuming ideal strong-base dissociation.
| Base | Formula | OH- per mole of base | [OH-] from 0.0075 M base | pOH | pH |
|---|---|---|---|---|---|
| Sodium hydroxide | NaOH | 1 | 0.0075 M | 2.1249 | 11.8751 |
| Potassium hydroxide | KOH | 1 | 0.0075 M | 2.1249 | 11.8751 |
| Calcium hydroxide | Ca(OH)2 | 2 | 0.0150 M | 1.8239 | 12.1761 |
| Strontium hydroxide | Sr(OH)2 | 2 | 0.0150 M | 1.8239 | 12.1761 |
| Barium hydroxide | Ba(OH)2 | 2 | 0.0150 M | 1.8239 | 12.1761 |
This table highlights a crucial lesson: if two strong bases have the same molarity but different numbers of hydroxide groups, their pH values are not necessarily the same. The pH depends on the actual hydroxide concentration released into solution, not just the written molarity of the base compound.
Concentration versus pH for Sr(OH)2
Another useful way to think about this problem is to compare several concentrations of strontium hydroxide. Because the pH scale is logarithmic, changes in concentration do not produce linear changes in pH. A tenfold dilution changes pOH by 1 unit and pH by 1 unit, assuming the same dissociation behavior and temperature reference.
| Sr(OH)2 concentration | [OH-] | pOH | pH at 25 C |
|---|---|---|---|
| 0.00075 M | 0.00150 M | 2.8239 | 11.1761 |
| 0.0010 M | 0.0020 M | 2.6990 | 11.3010 |
| 0.0050 M | 0.0100 M | 2.0000 | 12.0000 |
| 0.0075 M | 0.0150 M | 1.8239 | 12.1761 |
| 0.0100 M | 0.0200 M | 1.6990 | 12.3010 |
| 0.0500 M | 0.1000 M | 1.0000 | 13.0000 |
Common mistakes students make
- Ignoring the 2 in Sr(OH)2. This is the most frequent mistake and leads to an incorrect pH of about 11.88.
- Using pH = -log[base]. pH is based on hydronium concentration, not directly on the compound molarity. For bases, you typically calculate pOH first.
- Using natural log instead of log base 10. Chemistry pH formulas use common logarithms.
- Forgetting the temperature assumption. In standard chemistry exercises, pH + pOH = 14.00 is tied to 25 C.
- Rounding too early. Keep extra digits through the pOH step to avoid unnecessary rounding error.
Conceptual meaning of a pH near 12.18
A pH of 12.18 indicates a strongly basic solution. The pH scale is logarithmic, so a solution with pH 12.18 is not merely “a bit” basic. It has a hydroxide concentration far above neutral water. Neutral water at 25 C has pH 7 and [OH-] = 1.0 × 10-7 M. In this problem, the hydroxide concentration is 0.015 M, which is 1.5 × 10-2 M. Compared with neutral water, that is about 150,000 times greater in hydroxide concentration.
This is why even modest molar concentrations of strong metal hydroxides produce high pH values. The logarithmic scale compresses large concentration differences into a relatively small numeric pH range. Once students understand that point, values like 11.9, 12.2, and 13.0 become much easier to interpret.
When this simplified method works best
This calculator and the worked solution are ideal for introductory chemistry, AP Chemistry style practice, and general stoichiometric pH problems where the base is treated as fully dissociated. In that context, strontium hydroxide behaves as a strong base, and the method is straightforward.
In upper-level chemistry, you may need to think about additional factors such as ionic strength, activity coefficients, solubility limitations in certain setups, or temperature-dependent pKw values. Those effects matter in precision analytical work, but they are beyond the expected scope of most classroom pH exercises involving 0.0075 M Sr(OH)2.
Authoritative chemistry references
If you want to verify pH concepts, water chemistry fundamentals, and chemical safety details, these authoritative sources are useful starting points:
- USGS: pH and Water
- NCBI Bookshelf (.gov): Acids, Bases, and Buffers overview
- PubChem (.gov): Strontium Hydroxide compound data
Final answer
For the problem calculate the pH of 0.0075 M Sr(OH)2, the correct result at 25 C is:
[OH-] = 0.015 M
pOH = 1.8239
pH = 12.1761 ≈ 12.18
If you remember just one idea from this page, make it this: for metal hydroxides, always multiply the base molarity by the number of hydroxide ions released before taking the logarithm. That single check prevents the most common error and gives you the right pH every time.