What is the second member of the carboxylic acid series?

The second member of the carboxylic acid series is ethanoic acid. The carboxylic acids are a group of organic compounds that contain a carboxyl group (-COOH). These acids follow a homologous series, where each subsequent member differs from the previous one by a -CH2- unit.

Starting from the simplest carboxylic acid, which is methanoic acid (commonly known as formic acid), consisting of one carbon atom, the next member is ethanoic acid, which has two carbon atoms. This compound is more widely recognized as acetic acid, the main component of vinegar.

The options that include butanoic acid and propanoic acid are further along the series. Butanoic acid has four carbon atoms, and propanoic acid has three, making them not the second member of the series. Therefore, ethanoic acid is indeed the second member, as it follows methanoic acid in the sequence of carboxylic acids with the addition of one carbon atom, thereby retaining the essential -COOH functional group.