Which one of the following is the correct chemical formula of carboxylic acid?

Correct — C, C2H5COOH. A carboxylic acid is not a particular molecule but a family, and what makes a compound a member is the group written -COOH: a carbon carrying a doubled-bonded oxygen and a hydroxyl group on the same atom. Read the option that way and it declares itself — C2H5 is an ethyl group, COOH is the acid group, and the compound is propanoic acid, CH3CH2COOH. The hydrogen on that group is the one that comes away as H+ in water, which is what makes the family acidic; it is the same group that makes vinegar sour, since acetic acid is CH3COOH, and the same group that ants and nettles deliver as methanoic acid, HCOOH, the simplest member of all. What makes this item cleaner than it looks is that all four formulas describe three-carbon compounds. Nothing turns on chain length; everything turns on which group is hung on the end of the chain, which is exactly the point NCERT makes when it says a functional group confers its properties regardless of the length and nature of the carbon chain.

• (a)C3H7OH — Propanol, an alcohol. The group here is -OH alone, with no doubled-bonded oxygen on the same carbon. An alcohol is neutral to litmus; it takes the -COOH group to make an acid.
• (b)C3H8 — Propane, a saturated hydrocarbon with no functional group at all. It fits the alkane pattern CnH2n+2 and is the cooking-gas component, not an acid.
• (d)CH3COCH3 — Propanone, better known as acetone, a ketone. It does have a doubled-bonded oxygen, but that carbonyl carbon sits between two carbon atoms and carries no hydroxyl group. That single difference is what separates a ketone from an acid.

Carbon compounds are sorted by functional group — an atom or group of atoms substituted into a carbon chain that decides the compound's chemistry. NCERT's Class 10 table lists the important ones: halogens, then, for oxygen, alcohol (-OH), aldehyde, ketone and carboxylic acid (-COOH). The carboxylic acids are the organic acids of everyday life: methanoic acid HCOOH in ant stings and nettle hairs, ethanoic acid CH3COOH in vinegar, citric acid in lemons, and the long-chain fatty acids whose sodium salts are soap. A series of compounds sharing one functional group along chains of increasing length is a homologous series.

The safe method is to translate each formula into a group rather than trying to recall names. Look for -COOH: only one option has it. If a formula shows -OH by itself the compound is an alcohol; if it shows a carbonyl flanked by two carbons, as CH3COCH3 does, the compound is a ketone; and if it shows nothing but carbon and hydrogen in the pattern CnH2n+2, it is an alkane. A second check is available in the shape of the option itself: examiners write carboxylic acids with the acid group spelled out at the end, COOH, precisely so that it can be spotted. Note that the stem asks for 'the correct chemical formula of carboxylic acid', as though there were only one; there are many, and what the question really wants is the formula that belongs to the family.

• The carboxylic acid functional group is -COOH — a carbonyl and a hydroxyl on the same carbon atom.
• C2H5COOH is propanoic acid; the simplest member of the family is methanoic acid, HCOOH, and the most familiar is ethanoic acid, CH3COOH, the acid in vinegar.
• C3H7OH is propanol, an alcohol; C3H8 is propane, an alkane; CH3COCH3 is propanone or acetone, a ketone.
• All four options are three-carbon compounds, so the item turns purely on the functional group, not on chain length.
• A functional group confers its properties on the compound regardless of the length and nature of the carbon chain — which is what defines a homologous series.

• Confusing a ketone with an acid because both contain a carbonyl; the acid also carries an -OH on that same carbon.
• Reading -OH anywhere in a formula as evidence of an acid — C3H7OH is an alcohol and is not acidic.
• Being distracted by chain length when every option here has three carbons.

Either as formula-to-family recognition like this one, or the other way round — naming a compound such as acetone or ethanol and asking which family it belongs to.

• practice — not a real PYQ

  Which one of the following functional groups is present in ethanoic acid?

  • (a)-OH
  • (b)-CHO
  • (c)-COOH
  • (d)-CO-

  Answer(c) -COOH — ethanoic acid is CH3COOH, and the carboxyl group is what makes the compound acidic. The others are alcohol, aldehyde and ketone groups.

• practice — not a real PYQ

  CH3COCH3 belongs to which one of the following classes of carbon compounds?

  • (a)Alcohol
  • (b)Aldehyde
  • (c)Ketone
  • (d)Carboxylic acid

  Answer(c) Ketone — the carbonyl carbon lies between two carbon atoms and carries no hydroxyl, which is what makes propanone a ketone rather than an acid.