Question

A buffer contains 0.010 mol of lactic acid (pka = 3.86) and 0.050 mol sodium lactate per liter of aqueous solution.

a. Calculate the ph of this buffer.

Answer 1

The given buffer solution contains lactic acid and sodium lactate

i.e. CH3CH(OH)COOH/CH3CH(OH)COONa

Lactic acid is therefore a monoprotic acid with pKa = 3.86

Concentration of Lactic acid = 0.010 mol/L = 0.010 M

Concentration of sodium lactate = 0.050 mol/L = 0.050 M

Based on Henderson-Hasselbalch equation, the pH of the buffer is given as:

pH = pKa + log[salt]/[acid]

pH = pKa + log [CH3CH(OH)COONa]/[CH3CH(OH)COOH]

pH = 3.86 + log[0.050/0.010] = 4.56