pH of a 10.0M HCI solution

The pH of 10.0 M HCl cannot be found by calculation.

If you assumed that HCl is a strong acid which is completely ionized, the pHcould be simply calculated as follows :
pH = -log(10) = -1

The above calculation does NOT give a correct pH value of the solution. This is because the acid is too concentrated,and thus the hydrogen ion and chloride ions combine to give HCl molecules to a greatextent. In other words, the acid are onlypartially ionized, and the concentration of hydrogen ions CANNOT be assumed tobe equal to that of the acid.
HCl(aq) ⇌ H^+(aq) + Cl^-(aq)

Usually, no matter whether the acid is strong or weak, the calculation of thepH of an acid solution is only valid for dilute solutions.