What is the formula for pH?

pH = -log₁₀[H⁺]. A pH of 7 is neutral at 25°C; below 7 is acidic, above 7 is basic.

How are pH and pOH related?

At 25°C, pH + pOH = 14. This comes from K_w = [H⁺][OH⁻] = 10⁻¹⁴.

Yes. Very concentrated strong acids (such as 12 mol/L HCl) have negative pH. The formula -log[H⁺] gives a negative number when [H⁺] is greater than 1 mol/L.

Does pH + pOH = 14 always hold?

Only at 25°C. K_w increases with temperature, so at higher temperatures the sum is less than 14, and neutral pH is below 7.

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NSW · HSCChemistry

pH and pOH calculator

Enter any one of pH, pOH, [H⁺] or [OH⁻] and get the other three, plus an acidic/neutral/basic classification.

Inputs

InputpH

At 25°C, pH + pOH = 14 and K_w = [H⁺][OH⁻] = 10⁻¹⁴.

Result

3.000

pOH

11.00

[H⁺]

0.001000mol/L

[OH⁻]

1.000e-11mol/L

Classification

Acidic

pH = -log[H⁺], pOH = -log[OH⁻], pH + pOH = 14 (at 25°C).

How this calculator works

It applies the four standard relationships at 25°C: pH = -log[H⁺], pOH = -log[OH⁻], pH + pOH = 14, K_w = [H⁺][OH⁻] = 10⁻¹⁴. Whichever variable you enter, the calculator inverts to find the others.

Common questions

What is the formula for pH?: pH = -log₁₀[H⁺]. A pH of 7 is neutral at 25°C; below 7 is acidic, above 7 is basic.
How are pH and pOH related?: At 25°C, pH + pOH = 14. This comes from K_w = [H⁺][OH⁻] = 10⁻¹⁴.
Can pH be negative?: Yes. Very concentrated strong acids (such as 12 mol/L HCl) have negative pH. The formula -log[H⁺] gives a negative number when [H⁺] is greater than 1 mol/L.
Does pH + pOH = 14 always hold?: Only at 25°C. K_w increases with temperature, so at higher temperatures the sum is less than 14, and neutral pH is below 7.