Which one of the following is a weak acid
A weak acid in water produces H3O+ only partially and coexists with a substantial amount of the undissociated acid at equilibrium; a strong acid belongs to the small set that dissociates essentially completely in water.
Representative multiple-choice set
One standard interpretation in general chemistry is a single-correct option drawn from common strong acids and a typical weak acid:
Correct option: CH3COOH (acetic acid) is a weak acid.
HCl, HNO3, HClO4, and HBr are strong acids in water, so they do not remain largely undissociated under typical aqueous conditions.
Acid ionization language used in aqueous equilibrium
For a monoprotic acid written generically as HA, the ionization in water is described by \[ \mathrm{HA(aq)} + \mathrm{H_2O(l)} \rightleftharpoons \mathrm{H_3O^+(aq)} + \mathrm{A^-(aq)}. \]
The acid ionization constant is \[ K_a = \frac{[\mathrm{H_3O^+}]\,[\mathrm{A^-}]}{[\mathrm{HA}]}, \] with water treated as a pure liquid. A weak acid has a comparatively small \(K_a\), and equilibrium lies to the left; a strong acid has an equilibrium so far to the right in water that essentially all HA is converted to ions.
Strong-acid list used as a quick filter
A reliable screening criterion in introductory general chemistry is the common strong acids in water (at ordinary concentrations). When an option matches this list, it is not the weak acid.
| Common strong acids in water | Typical weak acids (examples) | Notes on interpretation |
|---|---|---|
| HCl, HBr, HI | HF, HCN, CH3COOH | Hydrogen halides: HCl/HBr/HI strong; HF weak due to strong H–F bond and solvation/ion-pairing effects. |
| HNO3 | HNO2 | Oxoacids: higher oxidation state and more O atoms generally increase acidity; nitrous acid remains weak. |
| HClO4 (and commonly HClO3) | HClO, HClO2 | Perchloric acid is strongly stabilized by resonance in ClO4−; hypochlorous acid is weak. |
| H2SO4 (first proton) | H2CO3, H3PO4 | Polyprotic acids can have mixed strength: the first dissociation of H2SO4 is strong; later dissociations are weaker. |
Equilibrium picture in water
Numerical scale for a typical weak acid
For a \(0.10\ \mathrm{mol \cdot L^{-1}}\) solution of acetic acid with \(K_a \approx 1.8 \times 10^{-5}\), the weak-acid approximation gives
\[ [\mathrm{H_3O^+}] \approx \sqrt{K_a \times C} = \sqrt{\left(1.8 \times 10^{-5}\right) \times 0.10} = \sqrt{1.8 \times 10^{-6}} \approx 1.34 \times 10^{-3}\ \mathrm{mol \cdot L^{-1}}. \]The pH estimate follows as \[ \mathrm{pH} \approx -\log_{10}\!\left(1.34 \times 10^{-3}\right) \approx 2.87, \] demonstrating that the solution is acidic while the fraction ionized is small compared with the initial \(0.10\ \mathrm{mol \cdot L^{-1}}\).
Common pitfalls
- Memorized strong-acid set in water: HCl, HBr, HI, HNO3, HClO4 (and commonly HClO3), H2SO4 (first proton).
- Polyprotic behavior: first dissociation can be much stronger than later dissociations.
- Context dependence: “strong” and “weak” are usually stated for aqueous solution at ordinary dilution; the solvent and concentration regime matter.
Under the representative options shown above, the weak acid is CH3COOH, since it partially ionizes and establishes a measurable acid–base equilibrium in water.