Most acidic and basis substances whose pH lie between the 0-14 range. Do not confuse the term acid strength with pH. The strength of an acid has to do with the percentage of the initial protons that are ionized. Strong acids are Hydrochloric, Hydrobromic, Nitric, Sulfuric, and Perchloric acids. On the other hand, there are certain acids that will not ionize very easily when added to water. These acids will only give up there protons to water with difficulty.
Weak acids can exhibit low pH readings just as strong acids.
Acid strength of weak acids has more to do with the ionization constant of the acid. The larger the value of K a the more Hydronium ion will be in the solution for a given initial concentration of the acid since the value of Ka must equal the ionization expression on the right side of the above equation and the Hydronium ion concentration is in the top portion of that expression. The percentage of ioninzation will therefore be larger. Strong acids have K a 's that approach infinity in value. On the other hand, pH has to do with the equilibrium concentration of hydronium ion.