Question

Determine the pH and the complete solution composition of the following system (added to 1 liter of pure water), based on the governing equations. Show all the simplifying assumptions you make, then solve the system of equations based on those assumptions:

0.01 M benzoic acid, HBz, and 0.1 M NaCl in water

Answer 1

Concentration of HBz = 0.01 M

Concentration of NaCl in water = 0.1 M

Dissociation of Benzoic acid in water is given as -

C₆H₅COOH <===> C₆H₅COO^- + H⁺ K_a = 6.46 x 10^-5

Lets take 1 L of HBz and 1 L of NaCl is poured into 1 L of pure water,

[HBz] = (1 x 0.01) / 3 = 0.0033 M

[NaCl] = (1 x 0.1) / 3 = 0.0333 M

Now HBz will react with NaCl present to give -

C₆H₅COOH + NaCl <===> C₆H₅COONa + HCl

0.01 0.1 0 0

0 0.99 0.01 0.01

New composition of solution,

[HBz] = 0

[NaCl] = 0.99 / 3 = 0.33 M

pH = -log [H⁺]

pH = -log [0.01]

pH = 2