Answer to Question #123872 in Physical Chemistry for Claudiu

Question

A buffered solution of 25 mL of an acid-base indicator is prepared by mixing 5mL of a stock solution of indicator 2,10x10^-4 M + 2,5 mL of Na2CO3 0,1 M + 10mL of NaHCO3 0,2 M
The stoichiometric equilibrium constant of carbonic acid are 4,47x10^-7 and 4,68x10^-11
Calculate the pH of this solution.

Answer 1

The acid-base equilibrium: HCO₃^- = H⁺ + CO₃^2-

Constant is 4.68*10^-11 = [H⁺]*[CO₃^2-]/[HCO₃^-]

Concentrations in resulting solution are:

[CO₃^2-] = 0.1*2.5/(5+2.5+10) = ‭0.014 M

[HCO₃^-] = 0.2*10/(+2.5+10) = ‭0.114 M

Therefore: [H⁺] = 4.68*10^-11 * 0.114 / 0.014 = 3.8*10^-10 M

pH = -log[H⁺] = 9.42