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Strong and weak acids

A strong acid, like HCl or HNO3, protolyzes (dissociates) completely in water, so $[\mathrm{H^+}]$ equals the stated concentration. A weak acid only partially dissociates, giving a lower $[\mathrm{H^+}]$ and higher pH than a strong acid at the same concentration.

[\mathrm{H^+}] = c \ \text{(strong acid, fully dissociated)}

for a strong acid of concentration

c

Symbols

c

stated (total) concentration of the acid

mol/L

Example

0.05 M HCl (strong acid): $[\mathrm{H^+}] = 0.05$ , $\mathrm{pH} = -\log(0.05) \approx 1.3$ .

For strong acids and bases you can set

[\mathrm{H^+}]

[\mathrm{OH^-}]

equal to the concentration directly — this does not hold for weak ones.

Practise acids, bases and equilibrium for free →

← The pH scale · Buffer →

Part of General Chemistry: Acids, bases and equilibrium.