Question

What is the pH at the EQUIVALENCE POINT of the titration of 0.100 M NaOH into a solution of 12.5 mL of 0.243 M butanoic acid

Answer 1

pH at equivalence point

At equivalence point we have all acid neutralized by base and only salt present

moles of acid = 0.243 M x 0.0125 L = 3.0375 x 10^-3 mols

Volume of base added = 3.04 x 10^-3/0.1 = 0.0304 L

molarity of salt in solution = 3.04 x 10^-3/(0.0125 + 0.0304) = 0.071 M

Salt hydrolyzes in water

A- + H2O <==> HA + OH-

let x amount has hydrolyzed then,

Kb = Kw/Ka = [HA][OH-]/[A-]

1 x 10^-14/1.51 x 10^-5 = x^2/0.071

x = [OH-] = 6.87 x 10^-6 M

pOH = -log[OH-] = 5.16

pH = 14 - pOH = 8.84