What a learner can do afterwards
- Calculates pH from the concentration of a strong acid and works backwards from a pH to a concentration
- Explains why a tenfold dilution changes pH by exactly one unit
- Uses the ionic product of water to get the hydrogen ion concentration in a strong alkali
- Explains why Kw changes with temperature and what that does to the pH of pure water
1 · Read
pH is a compact way to write a hydrogen ion concentration: pH = -log[H+]. A 0.01 mol per cubic decimetre HCl solution has pH 2, and working backwards, a pH of 3 means [H+] of 0.001 mol per cubic decimetre.
Dilute an acid tenfold and [H+] drops to a tenth of what it was. The log scale turns that tenth into a rise of exactly one pH unit, which makes dilution maths quick.
Water itself gives Kw = [H+] times [OH-] = 1.0 x 10-14 at 25 C. In 0.10 mol per cubic decimetre NaOH, [OH-] is 0.10, so [H+] is 1.0 x 10-13 and the pH works out as 13.
Kw grows as water warms, so pure water at 50 degrees has a pH below 7. It is still neutral, because [H+] and [OH-] stay equal to each other.
Logs compress concentrations to pH, Kw links the two ions, and warm pure water sits below pH 7 yet stays neutral.
2 · Watch
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.