what is the pKa of HNO2? What is the pKa of H2SO3? which of these is the weaker acid?
The correct answer and explanation is:
The pKa of nitrous acid (HNO2) is around 3.3, and the pKa of sulfurous acid (H2SO3) is approximately 1.9.
Sulfurous acid (H2SO3) is the stronger acid compared to nitrous acid (HNO2) because it has a lower pKa value. In general, the lower the pKa of an acid, the stronger the acid. This is because a lower pKa indicates that the acid dissociates more easily, releasing more hydrogen ions (H+) into the solution.
To understand why sulfurous acid is a stronger acid, consider the following:
- Acidity and pKa: The pKa value is a measure of the strength of an acid. A lower pKa indicates that the acid dissociates more readily in solution, producing a higher concentration of H+ ions. Sulfurous acid has a pKa of 1.9, indicating that it dissociates more effectively than nitrous acid, which has a pKa of 3.3.
- Structure of HNO2 (Nitrous Acid): Nitrous acid is a weak acid, and its dissociation is relatively less efficient. The conjugate base of HNO2, the nitrite ion (NO2-), is stabilized by resonance, but the acid itself does not dissociate as readily as stronger acids.
- Structure of H2SO3 (Sulfurous Acid): Sulfurous acid is stronger due to its ability to more easily release a proton (H+). Its conjugate base, the bisulfite ion (HSO3-), is stabilized by the electronegativity of the oxygen atoms and the resonance structure that spreads out the negative charge. The sulfur atom in sulfurous acid can more effectively stabilize the conjugate base, leading to stronger dissociation and a lower pKa.
In summary, H2SO3 is a stronger acid than HNO2 because it dissociates more readily, as evidenced by its lower pKa value. The strength of an acid is inversely related to its pKa: the lower the pKa, the stronger the acid.