how do we calculate pH value of a substance?

pH is a measure of the activity of dissolved hydrogen ions (H+),The formula for calculating pH is: Where αH+ denotes the activity(concentration) of H+ ions, and is dimensionless. Example:Dissolving the strong acid HCl in water can  be expressed as: HCl(aq) → H+ + Cl−This means that in a 0.01 mol/L solution of HCl it is approximated that there is a concentration of 0.01 mol/L dissolved hydrogen ions. From above, the pH is: pH = −log10 [H+]: pH = −log (0.01) which equals 2.