Quiz | Buffers and Acid-Base Titrations

A buffer solution is an aqueous solution consisting of a mixture of a weak acid and its conjugate base. Its pH changes very little when a small amount of strong acid or base is added to it and does not change when the solution is diluted

By definition:   K_a = $\frac{\left[{\mathrm{A}}^{-}\right] \left[{\mathrm{H}}_3{\mathrm{O}}^{+}\right]}{\left[\mathrm{HA}\right]}$  

If [HA] = [A^-], K_a = [H₃O⁺] ⇒ - log K_a = - log [H₃O⁺] ⇒ pK_a = pH

Titration of a strong acid with a strong base: pH changes abruptly at the equivalence point
At the equivalence point: stoichiometrically equivalent amounts of acid and base have reacted

Initial pH = 9.5 ⇒ weak base
Equivalence point: pH = 4.5 ⇒ Final pH ≈ 9.5 - 2 x 4.5 = 0.5 ⇒ strong acid

H₃PO₄ is a polyprotic acid: it can give up to 3 acidic proton:

H₃PO₄ (aq) + H₂O (l) ⇌ H₂PO₄^- (aq) + H₃O⁺ (aq) 
H₂PO₄^- (aq) + H₂O (l) ⇌ HPO₄^2- (aq) + H₃O⁺ (aq)
HPO₄^2- (aq) + H₂O (l) ⇌ PO₄^3- (aq) + H₃O⁺ (aq) 

3 equivalence points corresponding to the 3 previous acid-base equations are expected for the titration of H₃PO₄ by a strong base NaOH