Why is the pH scale 0-14?

Actually, the scale is not limited to 0-14, but most common fall into this range.

The pH of a solution is calculated as the negative base-10 logarithm of the hydronium ion (##H_3O^+##) concentration in solution.

Example 1: A 0.01 M solution of HCl (a strong acid that completely dissociates to ##H_3O^+## and ##Cl^-##) is given by pH = -log(0.01) = 2.0

Example 2: A 1.0 M solution of HCl has a pH of pH = -log(1.0) = 0.0

Example 3: A 2.0 M solution of HCl has a pH of pH = -log(2.0) = -0.30