pH & pOH of Strong Acids & Bases (College Board AP® Chemistry) : Study Guide

pH & pOH of Strong Acids & Bases

Strong Acids

• Most strong acids are monoprotic and fully ionize in aqueous solutions 

  • Examples include HCl (hydrochloric acid) and HNO₃ (nitric acid) 

• Due to the acid fully ionizing, there is a high concentration of  H⁺/H₃O⁺ ions resulting in the solution having a low pH 

• The H⁺/H₃O⁺ concentration is equal to the molar concentration of the acid:

  • [H⁺ (aq)] = M_{strong acid}

• This can be used to calculate the pH using:

  • pH = -log₁₀[H⁺ (aq)]

• The position of the equilibrium is so far over to the right that you can represent the reaction as an irreversible reaction

Diagram to show the ionization of a strong acid

The diagram shows the complete dissociation of a strong acid in aqueous solution

Strong Bases

• Strong bases, such as Group 1 metal hydroxides, completely dissociate in aqueous solutions

• A large concentration of OH^– ions are present in the solution causing it to have a high pH 

• Group 1 hydroxides produce one hydroxide ion per formula unit but others, such as Ca(OH)₂ will produce more than one 

• Therefore the concentration of hydroxide ions can be calculated using:

  • [OH^-] = M_{strong base} x number of OH^- ions per mole

• This can then be used to calculate pOH:

  • pOH = -log [OH^–]

• pOH can then be used to calculate pH:

  • pH = 14 - pOH

• The dissociation of a strong base  can be represented using the diagram below 

• The position of the equilibrium is on the right and so the reaction is irreversible 

Diagram to show the dissociation of a strong base

The diagram shows the complete dissociation of a strong base in aqueous solution