The pH of a solution is defined as the negative logarithm of the hydrogen ion (H+) concentration. We can use this definition to calculate the pH of a solution with a given [H3O+] concentration.
pH = -log([H3O+])
[H3O+] = 2.19 x 10^-5 M
pH = -log(2.19 x 10^-5)
pH = -log(2.19) - log(10^-5)
pH = -log(2.19) + 5
pH = 5 - log(2.19)
Approximating log(2.19) to 2 decimal places, we have:
pH = 5 - log(2.19) = 5 - 0.345 = 4.655
So the pH of the solution is approximately 4.655