Calculate the pH of 0.075 M KOH
Use this interactive calculator to find pOH, pH, and hydroxide concentration for potassium hydroxide solutions. The default example is 0.075 M KOH at 25°C, which is the classic strong-base calculation students see in general chemistry.
KOH pH Calculator
How to calculate the pH of 0.075 M KOH
To calculate the pH of 0.075 M KOH, start by recognizing that potassium hydroxide is a strong base. In aqueous solution, KOH dissociates almost completely into potassium ions and hydroxide ions. That means the hydroxide ion concentration is effectively equal to the starting molarity of KOH, assuming the solution is dilute enough for the standard general chemistry approximation. For a 0.075 M KOH solution, the hydroxide concentration is therefore 0.075 M. Once you have that value, you calculate pOH using the logarithmic relationship pOH = -log10[OH-]. After finding pOH, you convert to pH using pH + pOH = 14.00 at 25°C.
This type of problem is one of the most common strong-base calculations in chemistry. It appears in high school chemistry, AP Chemistry, first-year college chemistry, and laboratory report work. The reason it is so common is that it combines three key ideas at once: strong electrolyte dissociation, logarithms, and the relationship between pH and pOH. If you understand this one example well, you can use the exact same framework for NaOH, LiOH, RbOH, and other strong metal hydroxides that release one hydroxide ion per formula unit.
Step 1: Write the dissociation of KOH
Potassium hydroxide is a strong base and a strong electrolyte. In water, it dissociates as:
KOH(aq) → K+(aq) + OH-(aq)
Because this dissociation is essentially complete under standard introductory conditions, every mole of KOH contributes one mole of hydroxide ions. This is the core simplification that makes the calculation straightforward.
Step 2: Determine hydroxide concentration
If the concentration of KOH is 0.075 M, then:
- [KOH] = 0.075 M
- [OH-] = 0.075 M
There is a one-to-one stoichiometric relationship between KOH and OH-. Since each formula unit releases one hydroxide ion, the hydroxide concentration equals the molarity of the dissolved base.
Step 3: Calculate pOH
Use the formula:
pOH = -log10[OH-]
Substitute 0.075 for [OH-]:
pOH = -log10(0.075)
This gives:
pOH ≈ 1.1249
Step 4: Convert pOH to pH
At 25°C, the relationship is:
pH + pOH = 14.00
So:
pH = 14.00 – 1.1249 = 12.8751
Rounded appropriately:
pH ≈ 12.88
Final answer
The pH of 0.075 M KOH at 25°C is approximately 12.88.
Why KOH is treated as a strong base
Potassium hydroxide belongs to the alkali metal hydroxides, a family of compounds that dissociate extensively in water. In practical introductory calculations, KOH is assumed to dissociate completely. That is very different from weak bases such as ammonia, where only a fraction of the dissolved base reacts with water to produce hydroxide ions. For weak bases, you would need an equilibrium expression and a base dissociation constant. For KOH, you usually do not. That distinction matters because it removes equilibrium complexity and lets you jump directly from molarity to hydroxide ion concentration.
In real solutions, especially at higher ionic strength, activity effects can cause measured values to differ somewhat from idealized textbook calculations. However, for a standard classroom problem asking for the pH of 0.075 M KOH, the accepted method is the strong-base approximation shown above. This is the method your teacher, textbook, and most exam solutions will expect.
Common mistakes when solving for the pH of 0.075 M KOH
- Using pH directly from concentration. You must first calculate pOH because KOH gives hydroxide ions, not hydronium ions.
- Forgetting the negative sign in the logarithm. pOH = -log10[OH-], not log10[OH-].
- Subtracting in the wrong direction. At 25°C, pH = 14.00 – pOH, not pOH – 14.00.
- Assuming all bases are weak. KOH is not weak in standard general chemistry treatment.
- Mixing up one-to-one and multi-hydroxide stoichiometry. KOH gives one OH- per formula unit. Ca(OH)2 would give two.
Comparison table: pH values for common KOH concentrations at 25°C
The table below helps place 0.075 M KOH in context. All values assume complete dissociation and use pH + pOH = 14.00 at 25°C.
| KOH Concentration (M) | [OH-] (M) | pOH | pH at 25°C |
|---|---|---|---|
| 0.001 | 0.001 | 3.0000 | 11.0000 |
| 0.010 | 0.010 | 2.0000 | 12.0000 |
| 0.050 | 0.050 | 1.3010 | 12.6990 |
| 0.075 | 0.075 | 1.1249 | 12.8751 |
| 0.100 | 0.100 | 1.0000 | 13.0000 |
| 0.500 | 0.500 | 0.3010 | 13.6990 |
Temperature matters: pH and pOH are linked through pKw
Students are often taught the relation pH + pOH = 14.00, but that exact number applies at 25°C. More generally, the sum is equal to pKw, which changes with temperature. As temperature rises, the ion-product constant of water changes, and so does pKw. For many classroom questions, 25°C is implied unless stated otherwise. That is why the standard answer for 0.075 M KOH is 12.88. If your teacher or problem explicitly gives another temperature, use the appropriate pKw instead of assuming 14.00.
| Temperature | Approximate pKw | pOH for 0.075 M KOH | Approximate pH |
|---|---|---|---|
| 0°C | 14.94 | 1.1249 | 13.8151 |
| 10°C | 14.52 | 1.1249 | 13.3951 |
| 20°C | 14.17 | 1.1249 | 13.0451 |
| 25°C | 14.00 | 1.1249 | 12.8751 |
| 30°C | 13.83 | 1.1249 | 12.7051 |
| 50°C | 13.26 | 1.1249 | 12.1351 |
Detailed walkthrough using logarithms
Some students know the chemistry but get stuck on the math. Here is the logarithm part broken down more explicitly. The expression log10(0.075) is negative because 0.075 is less than 1. Specifically, log10(0.075) is about -1.1249. Since pOH is the negative of that logarithm, pOH becomes positive 1.1249. This is exactly why the minus sign in the formula matters. If you leave it out, you would get a physically meaningless negative pOH for this concentration range.
Next, subtract the pOH from 14.00 at 25°C. Since the pOH is low, the resulting pH is high, which makes sense because KOH is strongly basic. A pH near 12.88 indicates a highly alkaline solution. It is far more basic than neutral water, which is pH 7 at 25°C. The difference is large on a logarithmic scale, meaning the hydroxide concentration is many orders of magnitude higher than in pure water.
How this problem compares with acids and weak bases
If you were instead given 0.075 M HCl, the process would be similar but mirrored for a strong acid. You would treat [H3O+] as equal to 0.075 M, calculate pH directly, and obtain a low pH. If you were given 0.075 M NH3, the process would be more involved. You would need the Kb value for ammonia, set up an ICE table, solve an equilibrium expression, and then convert from pOH to pH. So while the KOH question looks simple, it teaches an important classification skill: identify whether the species is a strong acid, strong base, weak acid, or weak base before choosing your method.
Useful checklist for solving strong-base pH questions
- Identify whether the base is strong or weak.
- Write the dissociation equation.
- Determine how many hydroxide ions each formula unit produces.
- Set [OH-] equal to the stoichiometrically adjusted concentration.
- Calculate pOH using -log10[OH-].
- Convert to pH using pKw, usually 14.00 at 25°C.
- Round based on the precision of the given concentration.
Is 0.075 M KOH considered concentrated?
In an educational context, 0.075 M is a moderate molarity and is strong enough to produce a clearly basic pH without being near the extreme upper bounds seen in highly concentrated industrial solutions. It is sufficiently concentrated that the contribution of water autoionization is negligible relative to the hydroxide supplied by KOH. In other words, you do not need to add extra OH- from water because 0.075 M is overwhelmingly larger than 1.0 × 10-7 M.
That point is important because students sometimes worry that water itself contributes ions. It does, but in this case the effect is tiny compared with 0.075 M hydroxide from KOH, so it can safely be ignored in the standard calculation.
Practical interpretation of a pH near 12.88
A solution with a pH around 12.88 is strongly alkaline. Such a solution can be corrosive to skin and eyes and should be handled using proper laboratory safety practices, including gloves, splash goggles, and appropriate supervision where required. Potassium hydroxide is widely used in chemistry labs, soap manufacture, pH adjustment, and industrial cleaning processes. The high pH reflects the strong tendency of hydroxide-rich solutions to react with acids and with some materials they contact.
For environmental and water-quality context, most natural waters are nowhere near this pH. The U.S. Environmental Protection Agency discusses pH as a major water-quality parameter, and typical environmental waters fall in a much narrower band. This helps you understand just how strongly basic a 0.075 M KOH solution really is.
Authoritative references for pH and acid-base principles
- U.S. Environmental Protection Agency: pH overview
- University of Wisconsin Chemistry: acid-base concepts
- Purdue University Chemistry: acid-base equilibrium help
Bottom line
To calculate the pH of 0.075 M KOH, treat KOH as a strong base that fully dissociates in water. Set the hydroxide concentration equal to 0.075 M, calculate pOH as -log10(0.075), and then subtract from 14.00 at 25°C. The result is a pOH of about 1.1249 and a pH of about 12.8751, usually reported as 12.88. If a different temperature is specified, replace 14.00 with the appropriate pKw value.