pH and Kw
pH = −log[H⁺]. Water ionizes slightly: Kw = [H⁺][OH⁻] = 1.0×10⁻¹⁴ at 25 °C. So pH + pOH = 14. Each pH step is a factor of 10 in [H⁺].
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Analogy
A volume dial on a log scale: turn it by one notch and the loudness changes ten times.
Example
Strong acid, fully ionized[H⁺] = 0.010 MpH = −log(0.010) = 2.00pOH = 12.00
In practice
Blood must stay near pH 7.4, and a swing of even a few tenths is dangerous.
Practice
Flashcards
- What is Kw at 25 °C?1.0×10⁻¹⁴ = [H⁺][OH⁻].
Interview questions
- Calculate the pH of 0.10 M acetic acid (Ka = 1.8×10⁻⁵) and state the assumption you make.
- Set up the ICE table
- x²/0.10 = 1.8×10⁻⁵
- x ≈ 1.3×10⁻³ M, pH ≈ 2.87
- Assume x is small versus 0.10 (about 1.3 percent)
Questions people ask
Why is neutral water pH 7 only at 25 °C?
Water's ionization is endothermic, so Kw rises with temperature. At 50 °C, Kw is about 5.5×10⁻¹⁴, so neutral water has [H⁺] about 2.3×10⁻⁷ and a pH near 6.6. It is still neutral because [H⁺] equals [OH⁻]. Neutral means equal amounts, not a pH of exactly 7.
Can pH be negative or above 14?
Yes. Very concentrated strong acids can have [H⁺] above 1 M, giving a negative pH. Very concentrated strong bases can have a pH above 14. The 0 to 14 range is only the usual range for dilute solutions in water.