pH calculation formula:
  pH = -log(1/H⁺)
Where:
H⁺: Hydrogen ion concentration in the solution

H⁺ concentration of acid is depended on its pKa, for strong acid like HCl, its pKa=1, thus H⁺ concentration of 1 M HCl is also 1 M; for weak acid such as acetic acid, its pKa=0.0000175, thus H⁺ concentration of 1 M acetic acid is: 1 * 0.0000175 = 0.0000175 M