Analytical Chem istry - DePauw University

Chapter 6 Equilibrium Chemistry219A neutral solution has a hydronium ion concentration of 1.00 × 10 -7 Mand a pH of 7.00. For a solution to be acidic the concentration of H 3 O +must be greater than that for OH – , which means that+ −[ HO ] > 100 . × 10 7 M3The pH of an acidic solution, therefore, must be less than 7.00. A basicsolution, on the other hand, has a pH greater than 7.00. Figure 6.2 showsthe pH scale and pH values for some representative solutions.Ta b u l a t i n g Va l u e s f o r K aa n d K bA useful observation about acids and bases is that the strength of a base isinversely proportional to the strength of its conjugate acid. Consider, forexample, the dissociation reactions of acetic acid and acetate.+ −CH COOH( aq) + HO() l HO ( aq) + CH COO ( aq ) 6.123 2 3 3−−CH COO ( aq) + H O() l OH ( aq) + CHCOOH( aq ) 6.133 2 3Adding together these two reactions gives the reaction2H O() l H O + ( aq) + OH− ( aq )2 3for which the equilibrium constant is K w . Because adding together tworeactions is equivalent to multiplying their respective equilibrium constants,we may express K w as the product of K a for CH 3 COOH and K b forCH 3 COO – .K = K × K−w a,CH3COOH b,CH3COOFor any weak acid, HA, and its conjugate weak base, A – , we can generalizethis to the following equation.K = K × Kw a,HA − 6.14b,AThe relationship between K a and K b for a conjugate acid–base pair simplifiesour tabulation of acid and base dissociation constants. Appendix 11includes acid dissociation constants for a variety of weak acids. To find thevalue of K b for a weak base, use equation 6.14 and the K a value for its correspondingweak acid.Example 6.3Using Appendix 11, calculate values for the following equilibrium constants.(a) K b for pyridine, C 5 H 5 N(b) K b for dihydrogen phosphate, H 2 PO 4–pH1234567891011121314Gastric JuiceVinegar“Pure” RainNeutralSeawaterMilkBloodMilk of MagnesiaHousehold BleachFigure 6.2 Scale showing the pHvalue for representative solutions.Milk of Magnesia is a saturatedsolution of Mg(OH) 2 .A common mistake when using equation6.14 is to forget that it applies only to aconjugate acid–base pair.