When 3 moles of HCl is dissolved in 2 l of water is the resulting pH of the solution is -6.
The pH of a solution is related to the concentration of H+ ions as follows: pH = -log[H+]
As the pH of the solution created is -6, there are 10^6 H+ ions in the solution. HCl is a monoprotic acid and one molecule of HCl gives one ion of H+ and one Cl- ion. HCl --> H+ + Cl-
If the H+ concentration is 10^6, the Cl- concentration is also 10^6. The concentration of HCl in the solution is 3/2 = 1.5 M.
But the concentration of H+ cannot exceed 1.5 even if all of the acid dissolved is dissociated. There is an error in the information provided.
To determine the pKa of an acid use the relation pH = pKa + log([A-]/[HA]). It has to be noticed that [A-]/[HA] cannot be greater than 1.