Final answer:
The strong acid among the compounds listed is HCl (hydrochloric acid), which completely dissociates into ions in water.
Step-by-step explanation:
The question asks which of the listed compounds is a strong acid. Among HCOOH, H₂S, HF, HCN, and HCl, the compound that is recognized as a strong acid is hydrochloric acid, represented by the formula HCl. Strong acids are characterized by their ability to dissociate completely in water into their constituent ions. HCl, when dissolved in water, dissociates completely into hydrogen ions (H+) and chloride ions (Cl-), as shown in the following reaction:
HCl (aq) → H+(aq) + Cl-(aq)
Unlike strong acids, weak acids do not completely dissociate in water. Based on the information provided, hydrochloric acid (HCl) is the correct answer to the question regarding which compound is a strong acid.