Calculate the pH of 0.075 M KOH
Use this interactive chemistry calculator to determine the pH, pOH, and hydroxide ion concentration of a potassium hydroxide solution. The tool assumes KOH is a strong base that fully dissociates in water under standard general chemistry conditions.
KOH pH Calculator
Enter molarity in moles per liter. Example: 0.075
These bases release one hydroxide ion per formula unit.
For KOH, keep this at 1.
Choose how many decimals to show in the result.
This calculator uses the standard introductory chemistry assumption at 25 C.
Results
Ready to calculate.
For 0.075 M KOH at 25 C, the pH is expected to be strongly basic. Click Calculate pH to see the exact value, the pOH, and a visual chart.
Chart displays how pH changes for nearby strong base concentrations to help you place 0.075 M KOH in context.
How to calculate the pH of 0.075 M KOH
To calculate the pH of 0.075 M KOH, you use the fact that potassium hydroxide is a strong base. In water, KOH dissociates essentially completely into potassium ions, K+, and hydroxide ions, OH-. Because each formula unit of KOH produces one hydroxide ion, the hydroxide concentration is equal to the original molarity of the base. That means for a 0.075 M KOH solution, [OH-] = 0.075 M.
Once the hydroxide concentration is known, the next step is to calculate pOH using the logarithmic relationship:
pOH = -log10[OH-]
Substituting the value:
pOH = -log10(0.075) = 1.1249 approximately
At 25 C, pH and pOH are linked by the standard equation:
pH + pOH = 14
So:
pH = 14 – 1.1249 = 12.8751
Rounded appropriately, the pH of 0.075 M KOH is 12.88.
Final answer: For a 0.075 M KOH solution at 25 C, the pH is approximately 12.88. The solution is strongly basic.
Why KOH is treated as a strong base
Potassium hydroxide belongs to the family of Group 1 metal hydroxides that are considered strong bases in aqueous solution. In general chemistry, a strong base is one that dissociates almost completely in water. This simplifies calculations because you do not need an equilibrium table or a base dissociation constant for ordinary classroom problems. Instead, you can usually assume that the concentration of hydroxide ions produced equals the stoichiometric amount released by the dissolved base.
- KOH dissociates as KOH(aq) to K+(aq) + OH-(aq)
- The stoichiometric ratio is 1:1
- Therefore, 0.075 M KOH gives 0.075 M OH-
- That hydroxide concentration is then used to calculate pOH and pH
This is why KOH problems are often easier than weak base problems involving ammonia or organic amines. With weak bases, the concentration of OH- must be found from an equilibrium calculation. With KOH, the math is direct and fast.
Step by step solution for 0.075 M KOH
- Write the dissociation equation: KOH to K+ + OH-
- Recognize KOH is a strong base and fully dissociates
- Set hydroxide concentration equal to base concentration: [OH-] = 0.075 M
- Calculate pOH: pOH = -log10(0.075) = 1.1249
- Use the 25 C relationship: pH = 14 – 1.1249 = 12.8751
- Round to an appropriate number of decimal places: pH = 12.88
If your instructor emphasizes significant figures, note that the concentration 0.075 M contains two significant figures. Depending on the reporting standard used in your course, the pH may be written as 12.88 or 12.875, but most classroom answers are reported as 12.88.
What the answer means chemically
A pH of about 12.88 places this solution well into the basic range. On the pH scale, values above 7 are basic, values below 7 are acidic, and 7 is neutral at 25 C. Because the pH scale is logarithmic, a solution at pH 12.88 is not just a little basic. It has a high hydroxide ion concentration relative to neutral water.
Pure water at 25 C has a pH near 7 and an OH- concentration of 1.0 x 10-7 M. By contrast, 0.075 M KOH has an OH- concentration of 7.5 x 10-2 M. That means the hydroxide concentration is 750,000 times greater than in neutral water. This illustrates why KOH is considered a caustic and highly alkaline substance.
| Solution | [OH-] in M | pOH | pH at 25 C | Interpretation |
|---|---|---|---|---|
| Pure water | 1.0 x 10-7 | 7.00 | 7.00 | Neutral |
| 0.001 M KOH | 0.001 | 3.00 | 11.00 | Basic |
| 0.010 M KOH | 0.010 | 2.00 | 12.00 | Strongly basic |
| 0.075 M KOH | 0.075 | 1.125 | 12.875 | Very strongly basic |
| 0.100 M KOH | 0.100 | 1.00 | 13.00 | Very strongly basic |
Common mistake to avoid
The most common error in this problem is calculating pH directly from the KOH concentration. For bases, you should usually calculate pOH first, because the concentration of a base gives you hydroxide ion concentration, not hydrogen ion concentration. Only after finding pOH do you convert to pH using pH + pOH = 14 at 25 C.
Another mistake is forgetting the stoichiometric factor. KOH produces one OH- ion per formula unit, so the hydroxide concentration is the same as the molarity of KOH. However, a base such as Ca(OH)2 can release two hydroxide ions per dissolved unit, so the hydroxide concentration would be doubled if complete dissociation is assumed.
Formula summary for strong base pH calculations
- Strong base dissociation: base concentration converts directly to OH- concentration according to stoichiometry
- pOH formula: pOH = -log10[OH-]
- pH formula at 25 C: pH = 14 – pOH
- For KOH specifically: [OH-] = [KOH]
These three ideas are enough to solve nearly every introductory chemistry problem involving the pH of a strong monohydroxide base.
Worked comparison with other KOH concentrations
It can help to compare 0.075 M KOH with nearby concentrations. Because pH depends logarithmically on concentration, a moderate change in concentration does not produce a giant numerical change in pH. The pH still rises, but the increase is compressed by the logarithmic scale.
| KOH Concentration (M) | OH- Concentration (M) | pOH | pH | Change vs 0.075 M |
|---|---|---|---|---|
| 0.025 | 0.025 | 1.602 | 12.398 | 0.477 pH units lower |
| 0.050 | 0.050 | 1.301 | 12.699 | 0.176 pH units lower |
| 0.075 | 0.075 | 1.125 | 12.875 | Reference point |
| 0.100 | 0.100 | 1.000 | 13.000 | 0.125 pH units higher |
| 0.200 | 0.200 | 0.699 | 13.301 | 0.426 pH units higher |
How this connects to the pH scale
The pH scale is often introduced as a 0 to 14 range, but in concentrated solutions pH can sometimes extend slightly outside those textbook bounds. For most classroom examples in dilute aqueous solution, however, the 0 to 14 framework is appropriate. A pH of 12.88 tells you the solution is strongly alkaline and contains a much lower hydrogen ion concentration than neutral water.
Because pH is logarithmic, each one unit increase in pH corresponds to a tenfold decrease in hydrogen ion concentration. This means a solution with pH 12.88 is far more basic than a solution with pH 11.88, even though the numbers differ by only one unit.
Real world context for potassium hydroxide
KOH is widely used in laboratories and industry. It appears in chemical manufacturing, biodiesel production, alkaline batteries, soap making, and cleaning formulations. Its high basicity makes it useful, but it also means it must be handled with care. Solutions in this pH range can irritate or damage skin and eyes, and they can react with acids vigorously.
- Laboratory titrations and solution preparation
- Industrial pH adjustment processes
- Soap and detergent manufacture
- Battery electrolyte applications in some systems
- Organic synthesis and catalysis
If you are using a pH calculation in a practical setting, remember that measured pH may differ slightly from the ideal value because of temperature, ionic strength, activity effects, calibration quality, and instrument limitations. The calculator on this page provides the ideal classroom result under standard assumptions.
When the simple method is valid
The direct approach used here is valid when:
- The base is strong and dissociates completely
- The solution is dilute enough for introductory assumptions to remain reasonable
- The temperature is 25 C when using pH + pOH = 14
- You are solving a general chemistry style problem rather than doing advanced thermodynamic analysis
For 0.075 M KOH, all of these assumptions are usually acceptable in educational contexts. Therefore, using [OH-] = 0.075 M is entirely appropriate.
Quick check with scientific notation
You can also express 0.075 M as 7.5 x 10-2 M. This makes the logarithm easier to think about:
pOH = -log10(7.5 x 10-2)
Using log rules:
pOH = -(log10 7.5 + log10 10-2)
pOH = -(0.8751 – 2) = 1.1249
Then:
pH = 14 – 1.1249 = 12.8751
This confirms the same answer.
Authoritative references
If you want deeper background on pH, water chemistry, and acid-base concepts, these sources are useful:
- U.S. Geological Survey: pH and Water
- U.S. Environmental Protection Agency: pH Overview
- Purdue University Chemistry: pH and Acid-Base Review
Bottom line
To calculate the pH of 0.075 M KOH, first recognize that KOH is a strong base and fully dissociates, giving [OH-] = 0.075 M. Then compute pOH = -log10(0.075) = 1.1249. Finally, use pH = 14 – pOH at 25 C to obtain pH = 12.8751. Rounded to two decimal places, the pH is 12.88.
This result shows that 0.075 M KOH is a strongly basic solution. If you need a fast answer, remember the short version: 0.075 M KOH has pH about 12.88 at 25 C.