<P> Strong acids and bases are compounds that, for practical purposes, are completely dissociated in water . Under normal circumstances this means that the concentration of hydrogen ions in acidic solution can be taken to be equal to the concentration of the acid . The pH is then equal to minus the logarithm of the concentration value . Hydrochloric acid (HCl) is an example of a strong acid . The pH of a 0.01 M solution of HCl is equal to − log (0.01), that is, pH = 2 . Sodium hydroxide, NaOH, is an example of a strong base . The p (OH) value of a 0.01 M solution of NaOH is equal to − log (0.01), that is, p (OH) = 2 . From the definition of p (OH) above, this means that the pH is equal to about 12 . For solutions of sodium hydroxide at higher concentrations the self - ionization equilibrium must be taken into account . </P> <P> Self - ionization must also be considered when concentrations are extremely low . Consider, for example, a solution of hydrochloric acid at a concentration of 5 × 10 M. The simple procedure given above would suggest that it has a pH of 7.3 . This is clearly wrong as an acid solution should have a pH of less than 7 . Treating the system as a mixture of hydrochloric acid and the amphoteric substance water, a pH of 6.89 results . </P> <P> A weak acid or the conjugate acid of a weak base can be treated using the same formalism . </P> <Dl> <Dd> (Acid: HA ↽ − − ⇀ H + + A − Base: HA + ↽ − − ⇀ H + + A (\ displaystyle (\ begin (cases) (\ ce (Acid:)) & (\ ce (HA <=> H+ + A ^ -)) \ \ (\ ce (Base:)) & (\ ce (HA+ <=> H+ + A)) \ end (cases))) </Dd> </Dl>

Why is the p in ph lower case