`remember that an acid is something that donates hydrogen ions. so, when considering the formula hc2h3o2 (acetic acid), which of the hydrogens listed are the ones that ionize (fall apart into solution as hydrogen ions)

The H in the carboxyl group.

Explanation:

Acetic acid can be written as CH₃COOH, where -COOH is the functional group carboxyl, responsible for the acidity of organic acids. The H in the carboxyl group is the one that is donated in the acid reaction.

CH₃COOH(aq) + H₂O(l) ⇄ CH₃COO⁻(aq) + H₃O⁺

Acetic acid is a weak acid, so just a small fraction of the molecules undergo this reaction to donate their hydrogen.

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