Calculate the concentration of an acid at the equivalence point.

Question

Titrate 25.00 cm^3 of an acetic acid solution with 0.100 mol/dm^3 of sodium hydroxide solution. At the equivalence point, 15.24 cm^3 of the hydroxide solution has been consumed.

a) Calculate the concentration of acetic acid.

b) Is the solution acidic, neutral or basic at the equivalence point?

Answer 1

Acetic acid (CH₃COOH) is a weak acid.

Sodium hydroxide (NaOH) is a strong base.

The equivalence point is when moles of acid = moles of base

cm³ = ml and dm³ = L

CH₃COOH + NaOH ==> CH₃COONa + H₂O ... balanced equation

a) mols NaOH used at equivalence = 15.24 ml NaOH x 1 L /1000 ml x 0.100 mol/L = 0.00152 mols NaOH

mols CH₃COOH originally present = 0.00152 mols NaOH x 1 mol CH₃COOH/mol NaOH = 0.00152 mols CH₃COOH

concentration of CH₃COOH = 0.00152 mols / 25.00 ml x 1000 ml / L = 0.0608 M = 0.0610 M (3 sig. figs.)

b) At equivalence, all of the CH₃COOH has been converted to CH₃COONa, which is the salt of a weak acid and a strong base. The pH of such a salt will have a basic pH because upon hydrolysis, you get the weak acid and the strong base. CH₃COONa + H₂O ==> CH₃COOH + NaOH (basic solution)