Which acid is the strongest among the four?

• A. Acetic acid
• B. Phosphoric acid
• C. Nitric acid
• D. Sulfuric acid

Answer: (D)

The key idea is how readily an acid donates protons in water, which is quantified by its pKa; stronger acids have lower pKa and dissociate more completely. Acetic acid is a weak acid with a relatively high pKa, so it donates protons far less readily. Phosphoric acid is stronger than acetic for its first proton but still weaker overall than a truly strong acid. Nitric acid is a strong monoprotic acid, meaning it dissociates nearly completely in water. Sulfuric acid, on the other hand, is a strong diprotic acid: the first proton dissociates extremely readily (very low pKa), so it releases H+ almost completely, and the second proton adds further to the acidity. This combination makes the solution from sulfuric acid more acidic than that from nitric acid, and far more than the weak acids listed. So sulfuric acid is the strongest among them.