Standard deviation

I'm sorry if I posted in the wrong section. (If I did tell me what section it's supposed to go) Anyways, how do I find the standard deviation of something when I only know the average and the amount of "tests" there were.

Like take this question for example: 300 equal samples were taken from a lake. 225 of the samples were in the pH range from 5.5 to 6.5. The mean pH is 6.0. What is the standard deviation?

Also, if there happens to be a way, let me know if I can somehow do this on a calculator program (TI-83 Plus).

Thank you.