How to calculate moles from pH and Ka value...please help.

I want to know how to calculate it, not the answer. Thanks in advance! Here is the question:

What mass of benzoic acid, HC7H5O2, would you dissolve in 300.0mL of water to produce a solution with pH=2.95

HC7O5O2 + H20 <---> H30+ + C7O5O2-

thanks!!

In addition to what you have provided, you need the Ka value for benzoic acid: 6.4 x 10-5

First step: calculate the [H+] at pH 2.95
[H+] = 10^-pH
[H+] = 10^-2.95 = 1.122*10^-3

You now need to calculate the acid concentration from the Ka equation:
We can take a short cut if we assume that the dissociation of the acid is very small and we take the concentration of the undissociated acid in the solution as the actual acid concentration:
Ka = [H+] [C7H5O2-] / [HC7H5O2]
2.95 = (1.122*10^-5)² / [HC7H5O2]
[HC7H5O2] = (1.122*10^-5)²/ 2.95

You want to do the calculations yourself: solve the above, which will give you the molarity of the benzoic acid solution. Then from the molar mass of the acid, you calculate mass of acid required to make 300mL of solution.

From the pH, work out [H3O]+.

You know that [H3O]+^2 = Ka x [benzoic acid], so you can now work that out, too.

Then work out how many moles there would be in 300 ml, instead of 1,000, and multiply by the Mr of benzoic acid.

By the way, this acid is almost insoluble, so I doubt whether, in reality, you could dissolve enough to make a solution of pH 2.95...