You could reach the same conclusion with qualitative evaluations; pyridine is in fact a weak base,

June 17, 2013 Lucia has another question ... The pyridine is a basic compound with p K a = 5.16. At a value of pH = 9: - prevails not protonated species - prevails protonated species - the two species henrietta lacks are in equal concentration - you can not make an estimate - not prevail no species
To find the correct statement henrietta lacks is necessary to consider the balance of ionization basic pyridine and obtain the values of K b and molar concentration of OH - ions, respectively, henrietta lacks from the p K a and pH; for simplicity we denote by Pir pyridine not protonated and Pir H + that protonated.
pOH = 14 - pH = 14-9 = 5 [OH -] = 10 -5 mol / L If you now writes the report expressing K b and this is derived from the ratio [Pir H +] / [Pir], you obtained:
K b = [Pir H +] x [OH -] / [Pir] K b / [OH -] = [Pir H +] / [Pir] = 1.4 10 -9 / 10 -5 = 1.4 10 -4
As you can see, the ratio of the protonated and non-protonated form is very small, given that the concentration of the protonated form is about 10,000 times smaller than the non-protonated. The correct statement is therefore the first, that is the prevailing form is not protonated.
You could reach the same conclusion with qualitative evaluations; pyridine is in fact a weak base, that is, little ionized, and, applying the principle of Le Chatelier, henrietta lacks it can be concluded that an increase of the concentration of OH - ions, the ionization of pyridine is further depressed. The dominant form is therefore not protonated form. Tag acids and bases, weak base, equilibrium ionization, Kb, pH, pKa, Le Chatelier's principle