Which acid is the weakest among the four?

Which acid is the weakest among the four?

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

Answer: (A)

Understanding acid strength in water comes down to how readily an acid donates a proton, which is reflected by its pKa value. The smaller the pKa, the stronger the acid; the larger the pKa, the weaker the acid. Carbonic acid has a first-dissociation pKa of about 6.3, so it only weakly donates protons in aqueous solution. In contrast, nitric acid is a strong acid with a pKa around -1.4, sulfuric acid dissociates almost completely in its first step (pKa well below 0), and phosphoric acid has a first-dissociation pKa around 2.15—much lower than carbonic acid. Because of these much lower pKa values (or complete dissociation), those acids are significantly stronger than carbonic acid. Therefore, carbonic acid is the weakest among the four.

Understanding acid strength in water comes down to how readily an acid donates a proton, which is reflected by its pKa value. The smaller the pKa, the stronger the acid; the larger the pKa, the weaker the acid.

Carbonic acid has a first-dissociation pKa of about 6.3, so it only weakly donates protons in aqueous solution. In contrast, nitric acid is a strong acid with a pKa around -1.4, sulfuric acid dissociates almost completely in its first step (pKa well below 0), and phosphoric acid has a first-dissociation pKa around 2.15—much lower than carbonic acid. Because of these much lower pKa values (or complete dissociation), those acids are significantly stronger than carbonic acid.

Therefore, carbonic acid is the weakest among the four.