# 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⁺].

Chemistry (university) · Part 08: Acids, bases and electrochemistry · Card 2 of 9 · Last updated 2026-10-19

## Analogy

A volume dial on a log scale: turn it by one notch and the loudness changes ten times.

## Example

**0.010 M HCl**

Strong acid, fully ionized
[H⁺] = 0.010 M
pH = −log(0.010) = 2.00
pOH = 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.

## Related concepts

- [Weak acids and Ka](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-8/chemistry-university-part-8-concept-3)
- [Brønsted-Lowry acids and bases](https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-8/chemistry-university-part-8-concept-1)

Web page: https://studyvisually.com/learn/science/chemistry-university/chemistry-university-part-8/chemistry-university-part-8-concept-2
Study this card in the app: https://studyvisually.com/app/learn?category=chemistry-university&card=chemistry-university-part-8-concept-2