It depends on the concentration. pH is the negative log of the
concentration of H+ atoms in a solution (measured it molarity, mol/L).
Phosphoric acid (H3PO4) has 3 Hs in it, so for every molecule of acid
dissolved, there are 3H+ ions.
But Phosphoric acid isn't a strong
acid, so it doesn't completely dissolve in water. You can tell whether
an acid is strong or not by looking at the number of Os and Hs. If it
has 2 more Os than Hs, it is strong.
One must look up how
thoroughly the acid dissolves in water. By looking it up on
wolframalpha, I find that for every mole of H3PO4
In the solution,
1.06 moles of H+ are present. So if I take the -log1.06, I find the pH
of 1M phosphoric acid, which is 1.1.