Calculate the pH of a 0.055M Solution of KOH
Use this interactive chemistry calculator to find pOH, pH, and hydroxide concentration for potassium hydroxide solutions. The default example is a 0.055 M KOH solution at 25°C.
KOH pH Calculator
KOH → K+ + OH–
[OH–] = concentration of KOH
pOH = -log10[OH–]
pH = 14 – pOH
Visualization
This chart compares the current solution’s pH and pOH, and places your hydroxide concentration beside neutral water references for context.
For a strong base like KOH, dissociation is effectively complete in introductory chemistry calculations, so the hydroxide concentration is taken directly from the molarity.
How to Calculate the pH of a 0.055M Solution of KOH
If you need to calculate the pH of a 0.055M solution of KOH, the process is straightforward once you recognize one key fact: potassium hydroxide is a strong base. In water, KOH dissociates essentially completely into potassium ions and hydroxide ions. That means the hydroxide concentration in solution is approximately equal to the original KOH concentration. For a 0.055 M solution, the hydroxide ion concentration is therefore 0.055 M. From there, you calculate pOH first, then convert that value to pH.
The final answer at 25°C is pH ≈ 12.74. The corresponding pOH is about 1.26. This places the solution well into the basic range of the pH scale, which is exactly what you would expect for a moderately concentrated strong base. In practical chemistry, this type of calculation is frequently used in general chemistry labs, analytical chemistry, water treatment discussions, and acid-base titration problems.
Step-by-Step Solution
- Write the dissociation equation: KOH → K+ + OH–.
- Recognize that KOH is a strong base, so it dissociates nearly 100% in dilute aqueous solution.
- Set the hydroxide concentration equal to the KOH concentration: [OH–] = 0.055 M.
- Use the pOH formula: pOH = -log10(0.055).
- Calculate pOH: pOH ≈ 1.2596.
- Use the relation at 25°C: pH + pOH = 14.00.
- Calculate pH: pH = 14.00 – 1.2596 = 12.7404.
- Round appropriately: pH ≈ 12.74.
This is the classic route for solving strong base pH questions. Unlike weak bases, there is no need for an equilibrium table, no need for a Kb expression, and no need to solve a quadratic. The chemistry is simpler because the hydroxide source is already fully available in solution.
Why KOH Makes This Calculation Easy
Potassium hydroxide belongs to the family of strong alkali metal hydroxides. Strong bases such as KOH, NaOH, and LiOH are commonly treated as completely dissociated in water in standard chemistry coursework. This matters because it lets you connect concentration directly to hydroxide ion concentration. If the solution were instead ammonia or another weak base, you would need to account for partial ionization using an equilibrium constant.
Another reason this problem is simple is the one-to-one stoichiometry of KOH dissociation. Every formula unit of KOH produces one hydroxide ion. That means a 0.055 M KOH solution gives 0.055 M OH–. If you had a base that released two hydroxide ions per formula unit, the relationship would be different.
Detailed Math for 0.055 M KOH
Let us show the arithmetic in a format often expected in homework or exams:
Given: [KOH] = 0.055 M
Since KOH is a strong base: [OH–] = 0.055 M
Find pOH:
pOH = -log(0.055) = 1.2596
Find pH:
pH = 14.00 – 1.2596 = 12.7404
Answer: pH = 12.74
Depending on your instructor or lab manual, you may be asked to report two decimal places because the concentration 0.055 has two significant figures. In many educational settings, reporting pH as 12.74 is a good final format.
Reference Table for Strong Base Calculations
| KOH Concentration (M) | [OH–] (M) | pOH | pH at 25°C |
|---|---|---|---|
| 0.001 | 0.001 | 3.00 | 11.00 |
| 0.010 | 0.010 | 2.00 | 12.00 |
| 0.055 | 0.055 | 1.26 | 12.74 |
| 0.100 | 0.100 | 1.00 | 13.00 |
| 1.000 | 1.000 | 0.00 | 14.00 |
This table highlights a useful pattern: every tenfold increase in hydroxide concentration changes pOH by 1 unit and pH by 1 unit in the opposite direction. Because pH and pOH are logarithmic scales, concentration changes are not linear in pH terms.
How Basic Is a 0.055M KOH Solution?
A pH of 12.74 indicates a strongly basic solution. Neutral water at 25°C has a pH near 7.00, so this KOH solution is far more alkaline than ordinary water. In fact, comparing hydroxide levels directly gives a dramatic difference. Neutral water has [OH–] near 1.0 × 10-7 M, while this KOH solution has [OH–] = 5.5 × 10-2 M. That means the hydroxide concentration is about 550,000 times greater than in neutral water.
| Solution | Approximate pH | [OH–] (M) | Relative to Neutral Water |
|---|---|---|---|
| Neutral water at 25°C | 7.00 | 1.0 × 10-7 | 1× |
| 0.001 M KOH | 11.00 | 1.0 × 10-3 | 10,000× more OH– |
| 0.055 M KOH | 12.74 | 5.5 × 10-2 | 550,000× more OH– |
| 0.100 M KOH | 13.00 | 1.0 × 10-1 | 1,000,000× more OH– |
Common Mistakes Students Make
- Using pH instead of pOH first: For strong bases, calculate hydroxide concentration, then pOH, then convert to pH.
- Forgetting complete dissociation: KOH is strong, so [OH–] is taken directly from the molarity in standard problems.
- Typing the logarithm incorrectly: Make sure the calculator is using base-10 log, not natural log.
- Subtracting in the wrong direction: At 25°C, pH = 14 – pOH, not pOH – 14.
- Ignoring units: A value like 55 mM must be converted to 0.055 M before direct use if your formula expects molarity.
When the Assumption pH + pOH = 14 Applies
The familiar relationship pH + pOH = 14.00 is tied to water at 25°C. In more advanced chemistry, this total can vary with temperature because the ion-product constant of water changes. However, for nearly all general chemistry classroom problems asking you to calculate the pH of a 0.055M solution of KOH, the expected assumption is 25°C unless another temperature is given explicitly.
This is why our calculator labels the result as a 25°C calculation. It matches the standard educational convention used in textbooks, quizzes, and lab reports.
Why Significant Figures Matter
The concentration 0.055 M has two significant figures. In logarithmic calculations, the number of decimal places in the pH typically corresponds to the number of significant figures in the concentration value. Since 0.055 has two significant figures, a pH reported to two digits after the decimal, 12.74, is usually appropriate. If your instructor asks for more precision during intermediate work, you can keep extra digits until the final line and then round.
Real-World Context of Potassium Hydroxide
Potassium hydroxide is used in many industrial and laboratory settings. It appears in soap production, chemical manufacturing, alkaline battery chemistry, biodiesel processing, and pH control applications. Because it is strongly caustic, even moderate concentrations require proper safety handling. A 0.055 M solution is not as concentrated as stock laboratory KOH, but it is still strongly basic and capable of causing irritation or burns with sufficient exposure.
In analytical chemistry, strong bases like KOH and NaOH are often used for titrations and standardization exercises. Understanding how to translate concentration into pH is a foundational skill because it connects stoichiometry, dissociation, logarithms, and acid-base theory in one short problem.
Authoritative Chemistry References
For additional background on acid-base chemistry, pH, and chemical safety, consult these reliable sources:
- U.S. Environmental Protection Agency (EPA)
- LibreTexts Chemistry, hosted by higher education institutions
- PubChem, National Institutes of Health
Quick Recap
- KOH is a strong base.
- A 0.055 M KOH solution gives [OH–] = 0.055 M.
- pOH = -log(0.055) = 1.26.
- pH = 14.00 – 1.26 = 12.74.
If your goal is simply to calculate the pH of a 0.055M solution of KOH, the final value is 12.74 at 25°C. The broader lesson is just as important: strong bases convert directly to hydroxide concentration, and logarithms turn that concentration into pOH and then pH. Once you understand that sequence, you can solve a wide range of similar acid-base problems quickly and accurately.