The pH of a solution is a measure of its acidity or basicity and is defined as the negative logarithm (base 10) of the hydronium ion concentration (H3O+). To calculate the hydronium ion concentration from the given pH value of 9.85, we can use the formula:
[H3O+] = 10^(-pH)
Substituting the pH value into the formula:
[H3O+] = 10^(-9.85)
Using a calculator, we can calculate the value:
[H3O+] ≈ 1.26 x 10^(-10) M
Therefore, the hydronium ion concentration in the given aqueous solution with a pH of 9.85 at 25°C is approximately 1.26 x 10^(-10) M.