What is the difference between a strong acid and a weak acid

When an acid dissolves, it can donate a proton (H⁺) to water, forming H₃O⁺. The extent of this donation is measured by the acid dissociation constant Ka. A large Ka (≫1) means the reaction proceeds nearly to completion; a small Ka (≪1) means only a fraction of molecules donate a proton.

Complete vs. Partial Dissociation

The pH of an acid solution is linked to Ka by the expression $pH = -\log[H_3O+]$. For a strong acid, $[H_3O+] \approx C$ (the initial concentration), so pH ≈ -log C. For a weak acid, $[H_3O+] = \sqrt{K_a C}$ when C is small, giving a higher pH for the same concentration.

Main contrasts between strong and weak acids:

• Strong acids have Ka > 1, weak acids have Ka < 1
• Strong acids fully dissociate, weak acids reach an equilibrium
• Conductivity of strong acid solutions is higher
• pH of a 0.1 M strong acid ≈1, same concentration weak acid ≈2–3

Experimental way to tell strength:

1. 1Prepare a dilute solution of the acid
2. 2Measure electrical conductivity
3. 3Higher conductivity indicates more ions → stronger acid
4. 4Alternatively, titrate with a strong base and note the steepness of the pH jump near equivalence

Comparison of a typical strong and weak acid:

In practice, hydrochloric acid (HCl) and sulfuric acid (H₂SO₄) dissociate almost entirely, making them strong. Acetic acid (CH₃COOH), carbonic acid (H₂CO₃), and phosphoric acid (H₃PO₄) only partially dissociate, classifying them as weak acids.