A0.001 molar solution of weak acid has a ph = 4.0. what is the pka of the acid?
a. 4
b. 5
c. 6
d. 7
e. 8

A 0.001 molar solution of weak acid has a pH = 4.0,The pKa is :

B. 5

Explanation:

pH = It is negative logarithm of hydrogen ion concentration.

pH = -log([H+])

Ka = It is the dissociation constant.

pKa = -log(Ka)

The acid dissociates according to the equation given below:

Here , The concentration of A- and H+ will be same

So,

[H+] = [A-].....(1)

Now ,

pH = 4.0

pH = -log([H+])

4.0 = -log([H+])

- 4.0 = log([H+])        take antilog both side::"use scientific calculator"

From equation (1), [A-] =

[HA] = 0.001 M (given)

Insert the value of HA , H+ and A- in Ka

pKa = -log(Ka)

pKa = 5