A weak base is a base that, upon dissolution in water, does not dissociate completely, so that the resulting aqueous solution contains only a small proportion of hydroxide ions and the concerned basic radical, and a large proportion of undissociated molecules of the base.
Bases yield solutions in which the hydrogen ion activity is lower than it is in pure water, i.e., the solution is said to have a pH greater than 7.0 at standard conditions, potentially as high as 14 (and even greater than 14 for some bases).
The formula for pH is: Bases are proton acceptors; a base will receive a hydrogen ion from water, H2O, and the remaining H+ concentration in the solution determines pH.
Given its greater H+ concentration, the formula yields a lower pH value for the weak base.
Taking the logarithm of both sides of the equation yields: Finally, multiplying both sides by -1, we obtain: With pOH obtained from the pOH formula given above, the pH of the base can then be calculated from
Strong bases have smaller H+ concentrations because they are more fully protonated, leaving fewer hydrogen ions in the solution.
[2] The typical proton transfer equilibrium appears as such: B represents the base.
Calculate the pH and percentage protonation of a .20 M aqueous solution of pyridine, C5H5N.
[3] First, write the proton transfer equilibrium: The equilibrium table, with all concentrations in moles per liter, is This means .0095% of the pyridine is in the protonated form of C5H5NH+.