The larger the value of the pka, the weaker is the acid CHr Coh< CF2 COH< HC pKa=4.76 pKa=0 pKa=-7 Weak acid Very strong acid increasing acid strength In pure water at 25C, the concentrations of hy fronium and hydroxide ons are equal to 10M. Since the concentration of water in pure water is 55.5M. we can calculate the Ka for water H3O[OH](0)(10 18×1016 (555) pKa=15.7The larger the value of the pKa, the weaker is the acid. CH3 CO2 H ﹤ CF3 CO2 H ﹤ HCl pKa = 4.76 pKa = 0 pKa = -7 Weak acid Very strong acid increasing acid strength In pure water at 25℃, the concentrations of hyfronium and hydroxide Ions are equal to 10 -7 M. Since the concentration of water in pure water is 55.5 M, we can calculate the Ka for water Ka = [H3O + ] [OH - ] [H2 O] = (10 -7 )(10 -7 ) (55.5) = 1.8 ×10 -16 pKa = 15.7