Question

Indicate the stronger acid and provide the reason.

1) Hydrochloric acid (HCl); It has a lower pKa value.
2) Acetic acid (CH₃COOH); It has a higher pKa value.
3) Sulfuric acid (H₂SO₄); It has a lower pH value.
4) Nitric acid (HNO₃); It has a higher pH value.

Answer 1

Hydroiodic acid (HI) is the stronger acid when compared to nitric acid (HNO3) because it completely dissociates in anhydrous acetic acid, while HNO3 only partially dissociates.

Explanation:

When determining the stronger acid between hydroiodic acid (HI) and nitric acid (HNO3), one should consider their behavior in different solvents. Studies have shown that in anhydrous acetic acid, a 0.1 M solution of HI is completely dissociated, while HNO3 is only partially dissociated, indicating HI is the stronger acid.

Furthermore, all common strong acids like HNO3 have pKa values less than zero, which suggests a greater tendency to lose a proton than the H3O+ ion. However, the complete dissociation of HI in anhydrous acetic acid surpasses this, making HI the stronger acid in comparison to HNO3.

A higher pH value typically signifies a weaker acid, meaning that acids with lower pH values are stronger. Therefore, nitric acid having a higher pH value in a given scenario would suggest that it is not the stronger acid.