Chlorous Acid Formula
Chlorous acid is the oxoacid of chlorine derived from the chlorite oxyanion, ClO2−.
Rule connecting oxyanions and oxoacids
Oxoacid names are tied to the names of their oxyanions:
- An oxyanion ending in -ite forms an acid ending in -ous.
- An oxyanion ending in -ate forms an acid ending in -ic.
Step-by-step determination of the chlorous acid formula
The name chlorous acid indicates the corresponding oxyanion is chlorite: chlorite = ClO2−.
Chlorite has a single negative charge, so adding one proton (H+) produces the neutral acid molecule:
\[ \text{H}^+ + \text{ClO}_2^- \rightarrow \text{HClO}_2 \]
Therefore, the chlorous acid formula is HClO2.
Consistency check with the chlorine oxyacid series
Chlorine forms a well-known sequence of oxoacids where the name indicates the oxygen count. Chlorous acid sits between hypochlorous and chloric acids.
| Oxyanion name | Oxyanion formula | Acid name | Acid formula |
|---|---|---|---|
| hypochlorite | ClO− | hypochlorous acid | HClO |
| chlorite | ClO2− | chlorous acid | HClO2 |
| chlorate | ClO3− | chloric acid | HClO3 |
| perchlorate | ClO4− | perchloric acid | HClO4 |
Optional oxidation-state check (sanity check)
For a neutral molecule HClO2, assigning typical oxidation numbers \(+1\) for H and \(-2\) for O gives chlorine’s oxidation state \(x\):
\[ (+1) + x + 2(-2) = 0 \quad \Rightarrow \quad x = +3 \]
This is consistent with chlorous acid being the “middle” chlorine oxyacid in the series (higher than hypochlorous, lower than chloric/perchloric).
Visualization: Chlorine oxyacid ladder (oxygen count increases upward)
Common mix-up: chlorous acid is HClO2, not HClO3 (chloric acid) and not HClO (hypochlorous acid).