Functional group present in sulphonic acids is :

Solution

Chemistry ma’am · 3 steps

Picture H₂SO₄ with one OH swapped for R: what stays on sulphur is -SO₃H.

1. Start from H2SO4. Sulphuric acid is (HO)₂SO₂: one sulphur with two =O and two -OH. Replace one -OH by an alkyl or aryl group R and you get a sulphonic acid, R-SO₃H.
2. Count around sulphur. In -SO₃H, sulphur is bonded to R, to two =O and to one -OH. That gives 3 oxygens and 1 acidic hydrogen, as in benzenesulphonic acid, C₆H₅SO₃H.

   Option	Group	What it really is
   (a)	-SO₂⁻	sulphonyl group, with no -OH and so no acidic H
   (b)	-S(=O)-OH, that is -SO₂H	sulphinic acid, which has one oxygen too few
   (c)	-SO₃H	sulphonic acid: two =O and one -OH on S
   (d)	-OSO₃H, written as -SO₄H	hydrogen sulphate ester, where an O sits between C and S

3. So. The functional group of sulphonic acids is -SO₃H, option (c).

The idea to keep. Count the oxygens on sulphur. -SO₃H is sulphonic acid, -SO₂H is sulphinic acid, and in -OSO₃H (hydrogen sulphate) the carbon is joined to O, not to S.

Answer from the NTA final answer key. Solution by PrepWiser.