Question

Acids that dissociate essentially to completion are strong acids. An example is given below. The mols of HCl in the solution initially will essentially all convert to H3O+ and Cl- ; ie if you start with 10 mols of HCl, at equilibrium, you will have 10 mols of H3O+ and 10 mols of Cl-.

What would the pH be of a 0.004 M solution of HCl?

(Remember that pH = -log[H₃O⁺])

Answer 1

HCl is a strong acid, hence it will dissociate completely to form H+ and Cl- ions

Undergoing reaction

                       

Initial                0.004M       0           0

Final                   0           0.004M   0.004M

pH = -log[H3O+] = -log[H+] = -log[0.004] = log(1000) - log(4) = 3 - log(4) = 3 - 2log(2) = 2.39

Hence the pH of the solution will be equal to 2.39