[bassman]

I have now seen 4 different ways to calculate ph in aqueous solutioun. Is there a standard way that isn't so confusing. I have a sample of 5 g of HBr in a 100ml aqueous solutionI have K = (H+) (-OH) = 1.0 x 10-7 M  . do i neede to convert thr solution into a mole value and if so how then where do I plug and play in the formula ph = -log10 (H3O+) ? Please help,Thanx

[Borek]

In general you may calculate pH always the same way, but in most cases it is a way too difficult, so you are looking for simplifications, that why you end with different methods for different cases. Read this pH calculation introduction to learn more.

In your case you have just a solution of strong acid, it is enough to calculate molar concentration, and plug it into definition (which is equivalent of assuming that the acid is fully dissociated). Compare with this example of pH of strong acid calculation.

Finally, you may use this pH cheat sheet to select correct way of calculation, but note that you should learn basic concepts first.