Select calculation mode - Choose from pH, [H⁺], pOH, or [OH⁻] as your starting value.
2
Enter your value - Input the known value (pH between 0-14, or concentration in mol/L).
3
Click "Calculate" - Get all four values (pH, pOH, [H⁺], [OH⁻]) with step-by-step solutions.
🧪 Understanding pH Scale
The pH scale measures how acidic or basic a solution is, ranging from 0 to 14 at 25°C.
pH < 7: Acidic solution (higher [H⁺])
pH = 7: Neutral solution (pure water)
pH > 7: Basic (alkaline) solution (higher [OH⁻])
Each unit change in pH represents a tenfold change in hydrogen ion concentration. For example, pH 3 is 10× more acidic than pH 4, and 100× more acidic than pH 5.
pH stands for "potential of hydrogen" or "power of hydrogen." It measures the concentration of hydrogen ions (H⁺) in a solution.
Why does pH + pOH always equal 14?
At 25°C, the ion product of water (Kw) equals 1.0 × 10⁻¹⁴. Taking -log of both sides gives: -log[H⁺] + (-log[OH⁻]) = -log(10⁻¹⁴) = 14, so pH + pOH = 14.
Can pH be negative or greater than 14?
Yes, extremely concentrated strong acids can have pH below 0, and concentrated bases above 14. However, the standard pH scale (0-14) covers most practical applications.
How does temperature affect pH?
pH changes with temperature because Kw changes. At 25°C, Kw = 1×10⁻¹⁴. At 50°C, Kw ≈ 5.5×10⁻¹⁴, so neutral pH = 6.63. This calculator assumes 25°C.
What is the difference between a strong acid and a weak acid?
Strong acids (HCl, H₂SO₄) completely dissociate in water. Weak acids (acetic acid) partially dissociate. The same concentration of strong acid has lower pH than weak acid.
How do I measure pH in the lab?
Common methods: pH meter (most accurate), pH indicator paper (litmus paper), or universal indicator solution. pH meters need calibration with buffer solutions.