Question

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

Answer 1

Titrating sodium carbonate with hydrochloric acid
The carbonate ion, CO3 , is the conjugate base of the carbonic acid, H2CO3, 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) + H+(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