Let me explain why your stomach hurts. You see there are two theories about acids and bases and they are Arrhenius and Bronsted-Lowry. Arrhenius states that acids are compounds that donate H+. Bases in Arrhenius are compounds that donate OH-. Bronsted-Lowry states that an acid is a hydrogen donor and a base is a hydrogen acceptor.
The difference between Arrhenius and Bronsted-Lowry is that Arrhenius states that the acids always contain H+ and that bases always contain OH-. Whereas, Bronsted-Lowry claims that bases do not need to contain OH-.
An example of an Arrhenius acid is HCI and an Arrhenius base is NaOH. A Bronsted-Lowry base is NH3 an acid would be HCI. A Bronsted-Lowry base does not need OH-.
There are also conjugate Acids and Bases. H-A (acid)+ B(base) <=> H-B+(conjugate acid) + A-(conjugate base)
pH is the negative log of the concentration of H+ ions. To find pH of a 4.3x10^-2 M HCl solution you would use the formula pH=-log[H+] to get a pH of 1.4. To find the concentration of [H+] in a HCl solution with a pH of 2.0 you would use the formula [H+]=10^-pH to get a concentration of 0.010M. To find pOH of a 0.04 M HCl solution you would use the formula pOH=-log[OH-] to get a pOH of 12.6.