A-Level pH, dissociation constants and buffers: worked solution

Question

A pyridine buffer was proposed to keep the pH of an organic reaction mixture steady (pKb of pyridine = 8.8). Outline how you would make up 50 cm3 of a pyridine buffer at 25 °C, starting from a 1 mol dm-3 pyridine stock solution. Assume you also have 1 mol dm-3 stock solutions of the usual laboratory reagents together with suitable glassware.

A buffer needs comparable amounts of the weak base pyridine and its conjugate acid, the pyridinium ion. The conjugate acid has pKa = 14 - pKb = 14 - 8.8 = 5.2, so a buffer with equal amounts of the two gives pH = pKa = 5.2. Procedure: measure 25.0 cm³ of the 1 mol dm⁻³ pyridine (= 0.025 mol) into a beaker. Add 12.5 cm³ of 1 mol dm⁻³ hydrochloric acid (= 0.0125 mol); the HCl half-neutralises the pyridine, converting half of it to pyridinium chloride. This leaves 0.0125 mol pyridine and 0.0125 mol pyridinium ion (a 1:1 mixture). Transfer to a 50 cm³ volumetric flask and make up to the 50 cm³ mark with distilled water and mix. This gives a buffer of pH ~ 5.2.