Ethyl pentanoate is the ester . Its pentanoyl portion contains five carbons, including the carbonyl carbon, while its alcohol-derived ethyl group contributes two more.
Hydrolysis can yield pentanoic acid and ethanol, or a pentanoate salt under basic conditions. Tracking the two parts of the ester prevents confusion between carbon-chain identity and total molecular carbon count.
[ref]
Organic Chemistry by John McMurry. Chapter21, Carboxylic Acid Derivatives: ester identity and hydrolysis
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