pH poh Calculator
pH poh - Perform scientific calculations with precision and accuracy.
pH & pOH Calculator
Calculate solution acidity and basicity
Enter any one value to calculate the others.
Potential of Hydrogen
Potential of Hydroxide
in mol/L
in mol/L
pH Scale
Neutral
pH and pOH
pH and pOH are measures of the acidity or basicity of an aqueous solution. They are defined as the negative logarithm of the hydrogen [H⁺] and hydroxide [OH⁻] ion concentrations. At 25°C, the following relationships hold true:
- pH + pOH = 14
- [H⁺][OH⁻] = 1 x 10⁻¹⁴
Understanding pH and pOH
The Two Scales of Acidity and Basicity.
What is pH?
pH is a scale used to specify the acidity of an aqueous solution. It is a direct measure of the concentration of hydrogen ions (H⁺).
The pH scale is logarithmic, meaning each whole number change represents a tenfold change in acidity.
A low pH (0-6) indicates an acidic solution (high H⁺ concentration). A high pH (8-14) indicates a basic solution (low H⁺ concentration). A pH of 7 is neutral.
Example: Lemon juice (pH ≈ 2) is acidic. Soap (pH ≈ 10) is basic. Pure water (pH = 7) is neutral.
What is pOH?
Similarly, pOH is a scale used to specify the basicity of an aqueous solution. It is a direct measure of the concentration of hydroxide ions (OH⁻).
Like pH, the pOH scale is logarithmic.
A low pOH (0-6) indicates a basic solution (high OH⁻ concentration).
A high pOH (8-14) indicates an acidic solution (low OH⁻ concentration).
A pOH of 7 is also neutral.
Example:A solution of NaOH (a strong base) will have a very low pOH because the concentration of OH⁻ ions is high.
The Formulas for pH and pOH
The formulas are defined as the negative base-10 logarithm of their respective ion concentrations:
pH = -log[H⁺]
pOH = -log[OH⁻]
Where [H⁺] and [OH⁻] are the molar concentrations of hydrogen ions and hydroxide ions.
Because of the negative logarithm, a higher ion concentration results in a lower pH or pOH value.
Example:A solution with an [OH⁻] of 1 x 10⁻⁴ M has a pOH of -log(10⁻⁴) = 4.
The Key Relationship: The Ion Product of Water (Kw)
In any aqueous solution, pH and pOH are intrinsically linked through the ion-product constant for water (Kw).
Water auto-ionizes slightly (H₂O ⇌ H⁺ + OH⁻). At 25°C, the product of the ion concentrations is always a constant: [H⁺] * [OH⁻] = Kw = 1.0 x 10⁻¹⁴.
By taking the negative logarithm of this entire equation, we get a simple, powerful relationship:
pH + pOH = 14
Example:This equation is always true for any aqueous solution at 25°C. It means if you know the pH, you can instantly find the pOH, and vice versa.
Real-World Application: Water Treatment and Titrations
The pH and pOH relationship is fundamental in chemistry.
Water Treatment: To ensure water is safe to drink, its pH must be controlled. Sometimes a base (increasing OH⁻) is added to neutralize excess acidity (H⁺). The pH + pOH = 14 relationship is constant throughout this process.
Acid-Base Titrations: When titrating an acid with a base, chemists often monitor the pH. As OH⁻ is added, it neutralizes H⁺, causing the pH to rise. The pOH correspondingly falls. The equivalence point is where the moles of acid equal the moles of base.
Biochemistry: Biological systems, like our blood, are buffered to a specific pH. The concentrations of both H⁺ and OH⁻ are tightly controlled to maintain this balance.
Example:A swimming pool's water is tested for pH. If it's too acidic (low pH), a base like sodium bicarbonate is added. This increases the [OH⁻], lowers the pOH, and consequently raises the pH.
Key Summary
- **pH** measures the concentration of hydrogen ions ([H⁺]), while **pOH** measures the concentration of hydroxide ions ([OH⁻]).
- Low pH is acidic; low pOH is basic.
- Both are logarithmic scales defined by the negative log of the ion concentration.
- In any aqueous solution at 25°C, they are linked by the simple equation: **pH + pOH = 14**.
Practice Problems
Problem: A solution has a pH of 5.0. What is its pOH, and is the solution acidic or basic?
Use the relationship pH + pOH = 14 to solve for pOH. Determine acidity based on the pH value.
Solution: pOH = 14 - pH = 14 - 5.0 = 9.0. Since the pH is 5.0 (less than 7), the solution is **acidic**.
Problem: A cleaning solution has a hydroxide ion concentration [OH⁻] of 1.0 x 10⁻⁴ M. What is its pOH and its pH?
First, calculate the pOH using its formula. Then, use the relationship pH + pOH = 14 to find the pH.
Solution: pOH = -log(1.0 x 10⁻⁴) = 4.0. pH = 14 - pOH = 14 - 4.0 = 10.0. The solution is basic.
Frequently Asked Questions
Which scale is used more often, pH or pOH?
The pH scale is used far more commonly. By convention, scientists and professionals in various fields almost always refer to a solution's acidity or basicity using its pH value, even if it's a basic solution.
Does the pH + pOH = 14 rule always work?
This specific relationship holds true at a standard temperature of 25°C (298 K). The ion-product of water (Kw) is temperature-dependent, so at higher temperatures, Kw increases and the sum of pH and pOH will be slightly less than 14. However, for most general chemistry purposes, 14 is the standard value used.
How do I calculate the [H⁺] from the pH?
You use the inverse of the logarithm function. The formula is [H⁺] = 10⁻ᵖᴴ. Similarly, for pOH, the formula is [OH⁻] = 10⁻ᵖᴼᴴ.
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