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 α_H⁺ 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.