Question

Calculate the pH Values of the following two solutions 1.3 x10^-3 M HCl solution and 1.3 x 10^-3 M lactic acid solution in 0.001 M Sodium lactate salt.

Both have the same value just one is HCl solution & the other lactic acid solution

Answer 1

Given concentration of HCl = 1.3 x 10^-3 M

HCl is a strong acid Hence it completely dissociates to give 1.2 x 10^-3 M of H⁺

here the lactate salt has no buffering action with HCl.since it is a strong acid and also lactate is not the salt of HCl.

therefore p^H = - log [H⁺]

                 = - log [ 1.3 x 10^-3 ]

                 = 2.89

Given concentration of lactic acid = 1.3 x 10^-3 M

Lactic acid is a weak acid. It has a p^ka value = 3.8 Given the conc of sodium lactate salt is 0.001 M

therefore p^H = p^ka + log [salt]/[acid]             [ Henderson equation for the pH of weak acid + salt]

                   = 3.8 + log[ 0.001 / 0.0013]

                   = 3.8 - 0.11 = 3.69

Hence the answer is

pH of HCl solution = 2.89

pH of Lactic acid solution = 3.69