Calculate the pH of 10 mM weak acid (HA) dissolved in water.

Given:

pK_a = 4

Assume degree of dissociation is very small.

Solution:

Concentration:

C = 10 mM

= 0.01 M

K_a = 10^-4

For weak acid:

[H⁺] = √(K_aC)

= √(10^-4 × 10^-2)

= √(10^-6)

= 10^-3

Therefore:

pH = 3

Answer:

3