// CHEMISTRY L5 — ACID-BASE EQUILIBRIA
Strong, weak, and the maths of pH.
A strong acid fully ionises; a weak acid barely bothers. That single difference reshapes the pH — and Ka puts a precise number on it. Pick an acid, set the concentration, and compute its pH.
// FIG. 01 — THE pH CALCULATOR
STRENGTH{{ strength }}
Ka{{ kaLabel }}
pKa{{ pkaLabel }}
{{ formula }}
0714
pH =
{{ ph }}
[H⁺] ≈ {{ hplus }} mol/dm³
// STRONG vs WEAK
Don't confuse strong with concentrated. Strength is about how fully an acid ionises. A strong acid (HCl, H₂SO₄, HNO₃) ionises completely — every molecule gives up its H⁺ — so [H⁺] equals the concentration. A weak acid (ethanoic, carbonic) sets up an equilibrium and only a tiny fraction ionises, so [H⁺] is much smaller and the pH is higher. Switch between HCl and ethanoic acid above at the same concentration to see the pH jump by two whole units.
// Ka AND pKa
For a weak acid HA ⇌ H⁺ + A⁻, the acid dissociation constant is Ka = [H⁺][A⁻] ÷ [HA]. A bigger Ka means more ionisation — a stronger weak acid. Because the numbers are tiny and awkward, we usually quote pKa = −log Ka: now a smaller pKa means a stronger acid. For a weak acid, [H⁺] ≈ √(Ka × concentration), which is the sum running in the calculator above.
// BUFFERS — RESISTING CHANGE
A buffer is a solution that barely changes pH when small amounts of acid or alkali are added. It's made from a weak acid together with its conjugate base (e.g. ethanoic acid + sodium ethanoate). The weak acid mops up added OH⁻; the conjugate base mops up added H⁺ — so the equilibrium shifts to soak up the shock. Your blood is buffered by carbonic acid and hydrogencarbonate to hold its pH near 7.4; stray far from that and cells stop working. That's acid-base equilibria keeping you alive.
// REFERENCES & FURTHER READING
Cross-check against your own exam board's specification (AQA, Edexcel, OCR, SQA).