Define a strong acid and a weak acid, with examples and how to compare strength.

• A. Strong acids fully dissociate (HCl, H2SO4); weak acids partially dissociate (CH3COOH); strength judged by Ka and pH.
• B. Strong acids always have higher pH than weak acids.
• C. Strong acids have smaller Ka values than weak acids.
• D. Strong acids produce hydronium ions only in organic solvents.

Answer: (A)

Acid strength is determined by how completely an acid donates its protons to water. A strong acid dissociates essentially completely, giving rise to many hydronium ions and very little undissociated acid. A weak acid only partially dissociates, so both the undissociated acid and its ions are present at equilibrium.

That’s why strong acids like hydrochloric acid and sulfuric acid are described as fully dissociating in water. In contrast, acetic acid is a weak acid because it only partially dissociates, establishing an equilibrium with a much smaller amount of hydronium ions compared to the amount of undissociated acid.

To compare strength, chemists use the acid dissociation constant, Ka. A larger Ka means a stronger acid, because it reflects a greater tendency to lose a proton. Equivalently, a smaller pKa indicates a stronger acid. While the pH of a solution also changes with acidity, it depends on concentration as well as strength, so Ka (or pKa) is the more precise measure of intrinsic acid strength.

So the correct statement captures both the full versus partial dissociation and the way strength is quantified and compared using Ka (and, when convenient, pKa). The idea that stronger acids always have higher pH is incorrect (they produce more hydronium ions, lowering pH at the same concentration), and a smaller Ka would actually indicate a weaker acid. The claim about dissociation only in organic solvents is not how strength is defined for aqueous solutions.