Which statement about weak acids and bases is true?

Which statement about weak acids and bases is true?

• A. They fully dissociate in water.
• B. They do not fully dissociate in water.
• C. They have Ka = Kb.
• D. They are always strong electrolytes.

Answer: (B)

Partial ionization in water defines weak acids and bases. They establish an equilibrium between the undissociated molecules and the ions, so only a fraction dissociates at any given concentration. Because the extent of dissociation is limited by relatively small dissociation constants, the ions produced are few, and the solution conducts electricity only weakly. This is why the true statement is that they do not fully dissociate in water. The alternative that they fully dissociate describes strong acids and bases, not weak ones. The idea that Ka equals Kb is not generally correct; Ka and Kb are related by Ka × Kb = Kw (at 25°C, Kw ≈ 1.0 × 10^-14 for water), so they are connected but not equal in general. And the claim that they are always strong electrolytes is false, since weak electrolytes dissolve to form mostly neutral molecules and few ions.

Partial ionization in water defines weak acids and bases. They establish an equilibrium between the undissociated molecules and the ions, so only a fraction dissociates at any given concentration. Because the extent of dissociation is limited by relatively small dissociation constants, the ions produced are few, and the solution conducts electricity only weakly. This is why the true statement is that they do not fully dissociate in water. The alternative that they fully dissociate describes strong acids and bases, not weak ones. The idea that Ka equals Kb is not generally correct; Ka and Kb are related by Ka × Kb = Kw (at 25°C, Kw ≈ 1.0 × 10^-14 for water), so they are connected but not equal in general. And the claim that they are always strong electrolytes is false, since weak electrolytes dissolve to form mostly neutral molecules and few ions.