Strong and Weak Acids/Bases Chapter 19 Notes, part IV Strong and Weak Acids/Bases

Strong/Weak Acids and Bases Whether an acid or base is considered strong is based on how much they dissociate, or break down into ions, when in solutions.

Strong/Weak Acids and Bases A strong acid is completely ionized in aqueous solution. A weak acid is only partly ionized in aqueous solution. A strong base is completely ionized in aqueous solution. A weak base is only partly ionized in aqueous solution.

Strong/Weak Acids and Bases Strong acids include: Binary acids—HCl, HBr, HI Ternary acids—HNO3, HClO3, HClO4, H2SO4 Strong bases include: All hydroxides with metals in the first two groups on the periodic table except Be and Mg.

Dissociation Constants The acid dissociation constant tells how much an acid will break into ions in water, giving us an index for strength of an acid. For the reaction of an acid: HA + H2O D H3O+ + A- The formula is: Ka = [H3O+][A-] [HA]

Dissociation Constants Acids that have more than one hydrogen ion (diprotic and triprotic acids) will have more than one Ka. For example, H3PO4 D H+ + H2PO4- Ka = 7.5x10-3 H2PO4- D H+ + HPO42- Ka = 6.2x10-8 HPO42- D H+ + PO43- Ka = 4.8x10-13

Dissociation Constants Kb works the same way, except it measures dissociation of bases. For the equation: XOH D X+ + OH- The formula is: Kb = [X+][OH-] [XOH]

% Ionization Another way in which dissociation can be written is as a percent ionization, which tells you what percentage of the hydrogen ions will dissociate.

Practice Problem #1 A 0.100M solution of acetic acid (HC2H3O2) is only partially ionized. From measurements of the pH of the solution, [H+] is 1.34x10-3M. What is Ka for acetic acid?

Practice Problem #2 A 0.0250M solutions of HNO2 has a pH of 3.50. What is the Ka for nitrous acid?

Practice Problem #3 What is the pH of a 0.315M HMnO4 solution if its Ka is 1.52x10-5?