Analytical Chem istry - DePauw University

218 Analytical Chemistry 2.0Dissociation o f Wa t e rWater is an amphiprotic solvent because it can serve as an acid or as a base.An interesting feature of an amphiprotic solvent is that it is capable of reactingwith itself in an acid–base reaction.2H O() l H O + ( aq) + OH− ( aq )6.102 3We identify the equilibrium constant for this reaction as water’s dissociationconstant, K w ,K w 3= [ HO+ ][ OH− ] = 100 . × 10 −146.11which has a value of 1.0000 × 10 –14 at a temperature of 24 o C. The valueof K w varies substantially with temperature. For example, at 20 o CK w is 6.809 × 10 –15 , while at 30 o C K w is 1.469 × 10 –14 . At 25 o C, K w is1.008 × 10 –14 , which is sufficiently close to 1.00 × 10 –14 that we can usethe latter value with negligible error.An important consequence of equation 6.11 is that the concentrationof H 3 O + and the concentration of OH – are related. If we know [H 3 O + ] fora solution, then we can calculate [OH – ] using equation 6.11.Example 6.2What is the [OH – ] if the [H 3 O + ] is 6.12 × 10 -5 M?So l u t i o n−Kw100 . × 10[ OH ] = =+[ HO ] 612 . × 103−14−5= 163 . × 10−10Th e pH Sc a l epH = –log[H 3 O + ]Equation 6.11 allows us to develop a pH scale that indicates a solution’sacidity. When the concentrations of H 3 O + and OH – are equal a solution isneither acidic nor basic; that is, the solution is neutral. Lettingsubstituting into equation 6.11+ −[ HO ] = [ OH ]3K w 3and solving for [H 3 O + ] gives= [ HO ] = 100 . × 10+ 2 − 14[ HO ] = 100 . × 10 = 1.00×10+ −14 −73