Analytical Chem istry - DePauw University

Chapter 6 Equilibrium Chemistry241The mass balance equations are−2−010 M= H CO + HCO + CO2 3 3 3. [ ] [ ] [ ]+010 . M=[ Na ]and the charge balance equation is+ + − − 2−[ Na ] + [ HO ] = [ OH ] + [ HCO ] + 2×[ CO ]33 3Practice Exercise 6.9Write appropriate mass balance and charge balance equations for a solutioncontaining 0.10 M KH 2 PO 4 and 0.050 M Na 2 HPO 4 .Click here to review your answer to this exercise.6G.4 pH of a Monoprotic Weak AcidTo illustrate the systematic approach to solving equilibrium problems, let’scalculate the pH of 1.0 M HF. Two equilibrium reactions affect the pH.The first, and most obvious, is the acid dissociation reaction for HF+ −HF( aq) + HO() l HO ( aq) + ( aq )2 3FStep 1: Write all relevant equilibrium reactionsand equilibrium constant expressions.for which the equilibrium constant expression is+ −[ HO ][ F ]−= = 68 . × 10 4 6.35[ HF]K a3The second equilibrium reaction is the dissociation of water, which is anobvious yet easily neglected reaction2H O() l H O + ( aq) + OH− ( aq )2 3K w 3= [ HO+ ][ OH− ] = 100 . × 10 −146.36Counting unknowns, we find four: [HF], [F – ], [H 3 O + ], and [OH – ]. Tosolve this problem we need two additional equations. These equations area mass balance equation on hydrofluoric acidand a charge balance equationC HF= [ HF] + [ F− ] 6.37+ − −[ HO ] = [ OH ] + [ F ]6.383With four equations and four unknowns, we are ready to solve theproblem. Before doing so, let’s simplify the algebra by making twoassumptions.Step 2: Count the unique species appearingin the equilibrium constant expressions;these are your unknowns. You haveenough information to solve the problemif the number of unknowns equals thenumber of equilibrium constant expressions.If not, add a mass balance equationand/or a charge balance equation. Continueadding equations until the numberof equations equals the number of unknowns.