Question

I. Ethanol
II. Acetic acid
III. Phenol
IV. Acetonitrile
The acidity of compounds I-IV in water follows the order:
A. IV < I < III < II
B. I < II < III < IV
C. IV < I < II < III
D. IV < III < I < II

Answer 1

We must remember that the acidic solution has a greater value of concentration of ${H^ + }$ ions than that of the pure water whereas basic solution have lower value of concentration of ${H^ + }$ions than that of the pure water.

Complete answer:
We must know that the Acetic acid is $C{H_3}COOH$, phenol is ${C_6}{H_5}OH$ and ethanol is ${C_2}{H_5}OH$ and all they have a polar hydroxyl group $(O - H)$
And acetonitrile is chemically written as $C{H_3}CN$.
Firstly, let’s start by checking the acidic properties in between acetic acid and Phenol.
As we all know that acetic acid $C{H_3}COOH$ is a stronger acid than phenol this is because the negative charge is delocalized on two O-atom centers and three carbon centers whereas charge in phenol is delocalized on one O-atom and three carbon centers.
Then, we will check how Phenol is a stronger acid than Ethanol. In case of phenol, the negative charge is delocalized only on the one oxygen and carbon atoms. As in comparison to the phenoxide ion and ethoxide ion the phenoxide ion is considered to be more stable because the phenol has resonating structures.
Then, let’s consider acetonitrile it is less acidic in comparison to the alcohols and acids as it is a less stable conjugate base after the removal of a proton from it.
Finally, after considering all the options the acidity of the compounds in water follows the following order:
$IV{\text{ }} < {\text{ }}I{\text{ }} < {\text{ }}III{\text{ }} < {\text{ }}II$
So, the correct answer is “Option A”.

Note:
We must know that the strength of an acid is directly proportional to the stability of the acid's conjugate base. Also we can say an acid that has a more stable conjugate base will be increased acidic than an acid that has a less stable conjugate base.