Final answer:
The hydronium ion concentration [H3O+] for a solution with a pH of 12.45 is calculated using the inverse logarithm: [H3O+] = 10^(-12.45) M, which equals 3.5 x 10^-13 M when rounded to two significant figures.
Step-by-step explanation:
To find the hydronium ion concentration [H3O+] when the pH is 12.45, we use the formula pH = -log[H3O+].
This formula can be rearranged to solve for [H3O+] by taking the antilog (inverse logarithm) of the negative pH, which is antilog(-12.45).
So, calculating the hydronium ion concentration we get: [H3O+] = antilog(-12.45) = 10-12.45 M.
To express this with 2 significant figures, since the given pH has 4 decimal places, [H3O+] = 3.5 x 10-13 M.