Question

Match the following acids with their $p{K_a}$ values.

Acid	$p{K_a}$
a. Phenol	i. 16
b. p- Nitrophenol	ii. 0.78
c. Ethanol	iii. 10
d. Picric acid	iv. 7.1

The options are -

	a	B	c	d
(A)	iv	Ii	iii	i
(B)	ii	I	ii	iv
(C)	iii	I	iv	ii
(D)	iii	Iv	i	ii

Answer 1

Hint:. The Ka value is directly proportional to the acidity of the compound. The pKa value is negative of log of Ka. Thus, the pKa values give the basicity of the acid. Higher is the basicity of an acid, higher will be the pKa values.

Complete step by step answer:
The pKa values give the basicity of the acid. Higher is the basicity of an acid, higher will be the pKa values.
The Nitro groups are more acidic than phenols because the Nitro groups are electron withdrawing groups and thus, they increase the acidity of the molecule. The p- Nitrophenol has one nitro group at para to phenol and Picric acid has three nitro groups at ortho and para positions to phenol.
Thus, the picric acid is most acidic. So, it will have the least pKa value.
After picric acid will be the para Nitrophenol and then phenol.
The phenol and ethanol both are alcohols. Out of these phenols are more acidic than phenols. So, they will be least acidic and thus will have the highest pKa value.
So, the order of pKa values is Ethanol > Phenol > p- Nitrophenol > Picric acid
Thus, the values will be as -

Acid	$p{K_a}$
a. Phenol	iii. 10
b. p- Nitrophenol	iv. 7.1
c. Ethanol	i. 16
d. Picric acid	ii. 0.78

So, the correct answer is “Option D”.

(D)

a - iii

b - iv

c - i

d - ii

Note: It must be noted that nitro group is the electron withdrawing group. They can strongly attract the electrons toward themselves. As a result, the O-H bond gets weakened. So, the hydrogen can be easily donated. Thus, they are strong acids.