definition - increases the concentration of OH- ions when dissolved in water  

• Bronsted-Lowry definition - accepts proton from another substance
  • even applies to substances not dissolved in water
• Lewis definition - electron-pair donor
• pOH = -log [OH-]
  • pOH = 14 - pH
• strong base - completely dissociates, [OH-] = [base]
  • hydroxides of alkali metals and heavy alkaline earth metals
• weak base - partially dissociates
  • Kb = [OH-][conjugate acid] / [base]
  • [OH-] = [conjugate acid] when Kb is very small

Find the pH of 0.0030 M solution of ammonia  

• Kb = 1.8 x 10-5
• Kb = [OH-] [ammonium] / [ammonia] = x2 / 0.0030
• x = [(1.8 x 10-5) (0.0030)]1/2 = 2.3 x 10-4 = [OH-]
• pOH = -log [OH-] = 3.63
• pH = 14 - 3.63 = 10.37