Question

A solution is prepared using a sodium salt of an unknown diprotic base. The unknown diprotic weak base solution will be analyzed for concentration by titration with a strong acid. The unknown is placed in an Erlenmyer flask, and small, measured volumes of a standard strong acid solution are added until the final equivalence point is reached. Then the concentration of the base is calculated fromthe known volumes, the one known concentration and stoichiometry. All solutions are kept at room temperature.

A. Before the titration starts, is the solution in the flask acidic, neutral or alkaline?

B. Write a complete ionic equation for the complete (Bronsted-Lowry) acid-base neutralization reaction of a generic weak diprotic base B^2- with hydrochloric acid.

C. List the most commonform of each solute species that is present in the reaction mixture at the final equivalence point, and label each as a strong acid, weak acid, neutral, weak base, or strong base.

D. At the final equivalence point, will the overall solution be strongly acidic, weakly acidic, neutral, weakly basic, or strongly basic?

E. List the solute species that are present when more standard is added after the final equivalence point, and label each as either a strong acid, weak acid, neutral, weak base or strong base.

F. If extra titrant is added tto the flask after the final equivalence point has been reached, will the mixture be strongly acidic, weakly acidic, neutral, weakly basic, or strongly basic?

Answer 1

Consider the salt to be Na₂B.

A. The solution contains the weak base B^2- , pH is basic.

B.   

B^2- + 2HCl^- = H₂B + 2Cl^-

C. At the final EP the solution contains H₂B . By hydrolysis:

H₂B         +        H2O =           HB^-   +         HO^-

H₂B   weak acid

HB^-   amphoter (weak base in the above reaction)

H2O neutral ( acid in the above reaction)

HO^-   strong base

D. The solution contains the weak acid H₂B. pH<7.

E. The solution contains the excess of HCl and H₂B.

The species in solution are:

H₃O⁺ strong acid

Cl^- ineffective base

H₂B weak acid, can be considered not dissociated in the presence of the strong acid (Le Chetelier principle, common ion effect).

F. The pH will be strongly acidic ( the solution contains HCl in excess).