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WEAK BASES Basic Information 1) Weak bases are less than 100% ionized in solution. 2) Ammonia (formula = NH3) is the most common weak base example used by instructors. 3) The acquire H+ in aqueous solutions. 4) They have a small value for Kb 5) weak bases acquire hydrogen ions from water leaving hydroxide ex.
Name | Formula | Kb | Ammonia | NH3 | 1.8 x 10-5 | Methylamine | CH3NH2 | 4.38 x 10-4 | Ethylamine | C2H5NH2 | 5.6 x 10-4 | Diethylamine | (C2H5)2NH | 1.3 x 10-3 | Aniline | C6H5NH2 | 3.8 x 10-10 | Pyridine | C5H5N | 1.7 x 10-9 |
A typical pH problemCalculate the pH of a 0.20 M aqueous solution of pyridine, C5H5N. The Kb for C5H5N is 1.8 x 10-9 First, write the proton transfer equilibrium:
The equilibrium table, with all concentrations in moles per liter, is | C5H5N | C5H6N+ | OH- |
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initial normality | .20 | 0 | 0 |
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change in normality | -x | +x | +x |
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equilibrium normality | .20 -x | x | x |
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