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Acids, bases and pH

Where pH really comes from — and why a weak acid is not a dilute strong one.

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Explanation mode
-10246810121415Neutral 7.002.88

The scale runs past both ends. Concentrated strong acid goes below 0 and concentrated strong base above 14. Nothing here is clamped, because clamping teaches a limit that does not exist.

What an indicator would show

  • Methyl orangeRed · Changes between pH 3.1 and 4.4
  • LitmusRed · Changes between pH 5 and 8
  • Bromothymol blueYellow · Changes between pH 6 and 7.6
  • PhenolphthaleinColourless · Changes between pH 8.3 and 10
WeakpKa 4.76

Description of this simulation

0.1 mol/L CH3COOH has a pH of 2.88. It is a weak acid, so only 1.31% of it dissociates. A strong acid at the same concentration would reach pH 1 — a weak acid is not a dilute strong one. That is 1.31e-3 mol/L of hydrogen ions against 7.67e-12 mol/L of hydroxide, so the solution is acidic.

pH

2.88

Acidic

pOH
11.12
Hydrogen ions [H⁺]
1.31e-3 mol/L
Hydroxide ions [OH⁻]
7.67e-12 mol/L
Dissociated
1.31 %

If it were a strong acid

1.00vs 2.88

Same concentration, fully dissociated. The gap is the whole difference between weak and strong.

The √(Ka·c) shortcut

Solved exactly
2.8814
The √(Ka·c) shortcut
2.8785
Difference
0.0029

The shortcut assumes almost none of the acid dissociates. It is close for a weak acid at ordinary concentration and drifts when the acid is stronger or very dilute; this lab solves the quadratic instead.

What this model shows — and what it simplifies

Educational Model

A strong acid is solved from the full charge balance, not from the shortcut that hydrogen ion concentration equals the acid concentration — which is why 10⁻⁸ mol/L hydrochloric acid comes out at pH 6.98 here rather than the impossible pH 8 the shortcut predicts.

Where do we see this in real life?

Blood is held between pH 7.35 and 7.45 by a carbonate buffer, and a tenth of a unit either way is a medical emergency. Soil pH decides which crops will grow, and a swimming pool is dosed daily to stay near 7.4 so the chlorine keeps working.