# Question #81531

May 14, 2014

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 ${10}^{x}$ key on your calculator).

[H₃O⁺] = ${10}^{-} 4.25$ = 5.6 × 10⁻⁵ mol/L

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⁺].