28. Explain, in terms of equilibrium shifts, how this buffer mixture resists changes in pH when small quantities of both a strong acid and a strong base are added.

Question

The buffer mixture was made up of:
monosodium phosphate(NaH2PO4) and disodium phosphate (Na2HPO4)

Answer 1

H2PO4^- is the acid
HPO4^2- is the base.

In strong acid is added you have
HPO4^2- + H^+ ==> H2PO4^- ; i.e,, when a strong acid is added to the buffer mixture the weak base gobbles up the acid so the pH doesn't change much.

In strong base the rxn is
H2PO4^- + OH^- ==> HPO4^2- + H2O; i.e., the weak acid gobbles up the strong bsse in a netralization reaction and form water and the pH doesn't change much.