Chapter 14 · Acids and Bases
14.8Water: Acid and Base in One
10 min · two checks
Predict
What is [H₃O⁺][OH⁻] in water at 25 °C, even in a base?
The idea
- Use K_w to move between [H₃O⁺] and [OH⁻].
- Explain why neutral water has both ions.
Water autoionizes: 2 H₂O ⇌ H₃O⁺ + OH⁻, or more briefly H₂O ⇌ H⁺ + OH⁻. At 25 °C, K_w = [H₃O⁺][OH⁻] = 1.0 × 10⁻¹⁴. In pure water the two concentrations are equal, each 1.0 × 10⁻⁷ M. That is neutral. Add acid and [H₃O⁺] rises; [OH⁻] falls because the product is fixed. Add base and the reverse happens. A basic solution is not a solution with zero hydronium. It is a solution where hydroxide wins.
The relationship is a conversion. If [H₃O⁺] = 1.0 × 10⁻³ M, then [OH⁻] = 1.0 × 10⁻¹¹ M. Acidic means [H₃O⁺] > [OH⁻], which at 25 °C means [H₃O⁺] > 1.0 × 10⁻⁷. K_w changes with temperature, so the famous 10⁻¹⁴ and the neutral pH of 7 are 25 °C facts. Say so when you use them.
Keep these
- K_w = [H₃O⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25 °C.
- Neutral: both ions at 1.0 × 10⁻⁷ M.
- Acidic and basic solutions still contain both ions.