What species are present in HNO2?

Question

A bottle in the lab is labeled 0.1M nitrous acid HNO₂ (aq). Which of the following species are actually in the solution in this bottle (even if in small amounts)?

1. HNO₂
2. H₂O
3. OH^-
4. H₃O⁺
5. NO₂^-
6. NO₃^-

Answer 1

Nitrous acid (HNO₂), characterized by a pKa value of approximately 3.4, exhibits weak acidic properties. When dissolved in water, it undergoes partial ionization according to the equilibrium equation: HNO₂ ⇌ H⁺ + NO₂^-. In this process, only a portion of the nitrous acid molecules dissociates into hydronium ions (H⁺) and nitrite ions (NO₂^-), with the majority of the molecules retaining their undissociated state. Thus, a 0.1M nitrous acid HNO₂ (aq) labeled bottle is expected to contain the following:

HNO₂ (Nitrous Acid): present as the solute

H₂O: present as the solvent

H₃O⁺: present due to protonation of water

NO₂^- (Nitrite Ion): present due to the dissociation of Nitrous Acid: HNO₂ ⇌ H⁺ + NO₂^-