The hydrogen ion concentration of a pH 4.25 solution is 5.6 × 10⁻⁵ mol/L
The definition of pH is
pH = -log[H₃O⁺], so
-log[H₃O⁺] = 4.25 or
log [H₃O⁺] = -4.25
To calculate [H₃O⁺], we take the antilogarithm of both sides of the equation (the
Note: we don't need to know the identity of the acid or its concentration to calculate [H₃O⁺]. If we know just the pH, we can calculate the [H₃O⁺].