Question

Determine pH for a solution containing 0.7 M HBr and 1.4 M HC₃H₅O₂.

Answer 1

A strong acid is defined as one that dissociates completely in water (see the Related Questions link to the left for a list of the strong acids). That means for every mole of acid added, one mole of free H+ (or equivalently, H3O+) is present in the solution. The pH of solution is defined this way:
pH = -log10 [H3O+]

or in English, the pH is equal to the negative logarithm (in base 10) of the concentration of H3O+ in the solution (the concentration must be in units of Molarity (M), which is moles per liter).

So to find the pH of a strong acid solution all you need to know is the concentration of the solution. Let me give a couple of examples.

• If you have a 0.01 M solution of hydrochloric acid, HCl, what is the pH?

pH = -log (0.01) = 2

• If you have a 0.007 M solution of sulfuric acid, H2SO4, what is the pH?

Well, this is special because sulfuric acid is a (quite) strong diprotic acid in diluted solutions (below 0.1 M). Therefore the concentration of H3O+ is twice the molarity of H2SO4 because this holds 2 protons (H+) per mole.
So in a 0.007 M solution of sulfuric acid, H2SO4, the concentration of H3O+ is:
0.007