Question

Sketch a titration graph for the titration of 20.0mL of 0.20M sodium carbonate with 0.10M HCl.

Answer 1

Titrating sodium carbonate with hydrochloric acid
The carbonate ion, CO3 , is the conjugate base of the carbonic acid, H 2-
2CO3, a weak acid. It acts as two
successive Brønsted-Lowry bases, and so will accept two protons from a Brønsted-Lowry acid.
The two proton-accepting equilibria are:
CO32- (aq) + H+(aq) gives HCO3 (aq) Ka = Ka1
HCO3-(aq) H2CO3(aq) → H2O(l) + CO2(g) Ka = Ka2
(Note: the acid dissociation constants, Ka, refer to the proton loss reactions, i.e. the reverse of the
equilibria as written).
When sodium carbonate is titrated with a strong acid, the titration curve has two equivalence points.
These correspond to the adding of each successive proton to the base, and occur at pH 8.31 and pH 3.69
(see figure below).