What is the hydrogen ion concentration of a solution with ph = 8.25?

You just need to know that pH is the logarithm of the inverse of the concentration of hydrogen ion.

Mathematically this is: pH = log { 1/[H+] }

Then if you know the pH you can find [H+] using the properties of the logarithm function.

=> pH = - log [H+] = > [H+] = 10^ (-pH)

=> [H+] = 10 ^ (-8.25) = 5.62 * 10^-9

Answer: the concentration of hydrogen ion, [H+], is 5.62 * 10^-9 M