Carboxylic acids with lowest molecular mass have shortest carbon chains

Steps:

• Identify the general formula for carboxylic acids: R-COOH, where R is a hydrocarbon chain.
• Calculate relative molecular mass (Mr) for each option using atomic masses (C=12, H=1, O=16).
• Compare Mr values: smallest chain yields lowest Mr.
• Select the structure with the shortest chain (fewest carbons).

Why A is correct:

• A represents formic acid (HCOOH), with Mr = (1×1 + 1×12 + 2×16 + 1×1) = 46, the smallest possible for a carboxylic acid per IUPAC definition.

Why the others are wrong:

• B (acetic acid, CH3COOH) has Mr=60 due to extra carbon, higher than A.
• C (propanoic acid, C2H5COOH) has Mr=74 from longer chain.
• D (butanoic acid, C3H7COOH) has Mr=88, largest chain shown.

Final answer: A