CHM 123 Weak Acids, Weak Bases and Buffers

CHM 123 Weak Acids, Weak Bases and Buffers

A.  Weak acids,

•  Only a few molecules dissociate.

•  Most of the weak acid remains as the undissociated (molecular) form of the acid.

•  The concentrations of the H3O+and the anion (A−) are small.

HA(aq)+ H2O(l) H3O(aq)+ A−(aq)

E.g Write an acid dissociation of HC2H3O2 in water

B.  Weak bases

•  are most other bases.

•  dissociate only slightly in water.

•  form only a few ions in water.

NH3(g) + H2O(l) NH4+(aq)+ OH−(aq)

C.  Common-Ion Effect:

The shift in the position of equilibrium on addition of a substance that provides an ion in common with one of the ions already involved in the equilibrium

HOCl(aq)+ H2O(l) H3O+(aq)+ -OCl(aq)

What are the ions presented in this acid solution?

In which direction will equilibrium shift if small amount of –OCl was added?

E.g Calculate the pH of 0.15M HF and 0.25NaF mixture.

D.  Buffer Solution:

A solution which contains a weak acid and its conjugate base and resists drastic changes in pH.

•  typically has an equal concentration of a weak acid and its salt.

•  may also contain a weak base and a salt of the conjugate acid.

Weak acid + conjugate base OR Weak base + Conjugate acid

E.g Which combination(s) make a buffer solution?

A. HCl and KCl

B. H2CO3 and NaHCO3

C. H3PO4 and NaCl

In the acetic acid/acetate buffer with acetic acid

(HC2H3O2) and sodium acetate (NaC2H3O2)

•  the salt produces acetate ions and sodium ions.

NaC2H3O2(aq) à C2H3O2− (aq) + Na+(aq)

•  the salt is added to provide a higher concentration of the conjugate base C2H3O2− than the weak acid alone.

HC2H3O2(aq)+ H2O(l) C2H3O2- (aq)+ H3O+(aq)

Large amount Large amount

Adding small amount –OH

HC2H3O2 (aq) + OH− (aq) à C2H3O2− (aq) + H2O(l)

acetic acid base acetate ion water

Adding small amount H3O+

C2H3O2− (aq) + H3O+(aq) à HC2H3O2 (aq) + H2O (l)

acetate ion acid acetic acid

The Henderson-Hasselbalch Equation

HA(aq) + H2O(l) H3O+(aq) + A-(aq)

E.g Calculate the pH of a buffer solution that is 0.50 M in benzoic acid (HC7H5O2) and 0.150 M in sodium benzoate (NaC7H5O2). Ka = 6.5 x 10-5

E.g Calculate the pH of 0.100L of a buffer solution that is 0.25M in HF and 0.50 M in NaF. Assume the volume remains constant and Ka = 3.5 x 10-4

a. What is the change in pH on addition of 0.002 mol HCl

b. What is the change in pH on addition of 0.010 moles KOH

Dang 1