October 11, 2025

What Is The Relationship Between Ph And Poh

Q: Is the relationship pH + pOH = 14 always true?

The relationship pH + pOH = pKw is always true. However, pKw is temperature-dependent, so the sum equals 14 specifically at 25°C; at other temperatures, pKw will have a slightly different value.

Q: What is the pH and pOH of a neutral solution?

At 25°C, a neutral solution (like pure water) has equal concentrations of H⁺ and OH⁻ ions. This means both its pH and pOH are equal to 7.

Q: Why are both pH and pOH scales used?

Both scales provide essential information about solution acidity or basicity. While pH is more commonly cited, pOH is particularly useful when focusing on highly basic solutions or calculations primarily involving hydroxide ion concentrations.

Q: How do strong acids or bases affect this relationship?

Strong acids and bases dissociate completely, significantly altering the [H⁺] or [OH⁻] concentrations. While they shift the pH and pOH to extreme values, the fundamental relationship pH + pOH = 14 (at 25°C) still holds for the resulting solution.

Explore the fundamental inverse relationship between pH and pOH in aqueous solutions, crucial for measuring acidity and alkalinity in chemistry.

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The Ion Product of Water (Kw)

In any aqueous solution, water molecules undergo a slight autoionization, producing both hydrogen ions (H⁺) and hydroxide ions (OH⁻). The product of their molar concentrations, [H⁺][OH⁻], is a constant called the ion product of water (Kw). At a standard temperature of 25°C, Kw is approximately 1.0 x 10⁻¹⁴.

Definitions of pH and pOH

pH is a logarithmic measure of the hydrogen ion concentration, defined as pH = -log₁₀[H⁺]. Similarly, pOH is a logarithmic measure of the hydroxide ion concentration, defined as pOH = -log₁₀[OH⁻]. These scales provide a convenient way to express very wide ranges of ion concentrations.

The Fundamental Relationship

Given the constant Kw, taking the negative logarithm of the ion product equation yields the core relationship: -log₁₀([H⁺][OH⁻]) = -log₁₀(Kw). This simplifies to -log₁₀[H⁺] + (-log₁₀[OH⁻]) = -log₁₀(Kw), or simply pH + pOH = pKw.

Practical Application (at 25°C)

At 25°C, where Kw is 1.0 x 10⁻¹⁴, the value of pKw is -log₁₀(1.0 x 10⁻¹⁴) = 14. Therefore, for most practical chemistry applications at this temperature, the sum of pH and pOH for any aqueous solution is always 14 (pH + pOH = 14). This allows for easy conversion between pH and pOH values.

Frequently Asked Questions

Is the relationship pH + pOH = 14 always true?

What is the pH and pOH of a neutral solution?

Why are both pH and pOH scales used?

How do strong acids or bases affect this relationship?