1. Write out the expression that defines pH
2. Assume HClO4 is close to 100% dissociated in solution
3. From the original concentration of HClO4, find [H+] and calculate the original pH from this
4. Calculate the new (diluted) concentration of HClO4 (molarity = number of moles/volume of solution)
5. Using the new value of [H+], calculate pH again.
6. Subtract from the original number to find the difference.