8/7/21, 10:24 AM6.7: Weak Acid or Weak Base Equilibria: Foundations of General Chemistry with Lab-2021- SchirenPage 1 of 12?module_item_id=1041366.7: Weak Acid or Weak Base EquilibriaThere are many weak acids and weak bases that ionize only partially by equilibrium reactions.One of the most common weak acids is acetic acid whose structural formula is shown below:The molecular formula, HCHO, is usually used for acetic acid when writing the equilibriumionization reaction:The equilibrium constant expression for the dissociation, K , is written as followsHCHOdissociates to an extent of about 1% in water, so we know that [HO] is a constant andcan be ignored in the expression producing the following known as K, the acid ionization constantin which we also simplify [HO] as [H]:The value of Kis determined experimentally by measuring the pH of a solution of HCHOofknown starting molarity. This is used to calculate [H] from which [CHO], and the equilibrium[HCHO] can be determined. In the case of [HCHO], K= 1.8 x 10. The Kof some commonweak acids is shown in the table below:AcidFormulaKAcetic acidHC H O1.8 X 10Benzoic acidHC H O6.3 X 10232c2322a3++a232+232-232232a-5aa232-5752-5
8/7/21, 10:24 AM6.7: Weak Acid or Weak Base Equilibria: Foundations of General Chemistry with Lab-2021- SchirenBenzoic acidHC H O6.3 X 10Cyanic acidHOCN3.5 X 10Formic acidHCHO1.7 X 10Hydrofluoric acidHF6.8 X 10Hypochlorous acidHOCl3.5 X 10Nitrous acidHNO4.5 X 10Propionic acidHC H O1.3 X 10Pyruvic acidHC H O1.4 X 10Examples:Determine the value of Kfor acetic acid (HCHO), if the pH of an 0.135 M solution is 2.81.