Question...

From Exam 3:
2. Explain why each of the following does not result in an effective buffer? (15pts)
1.24mol NH₄NO₃(aq) + 0.03mol NH₃(aq)
0.38mol HI(aq) + 0.38mol NaI(aq)
1.28mol Na₂CO₃(aq) + 0.64mol NaOH(aq)

What makes an effective buffer solution? 1) an approximately equimolar combination of a weak conjugate acid and its weak conjugate base; 2) at concentrations at least 100x the K_a of the weak conjugate acid.
The first combination listed above is indeed a combination of a weak conjugate acid (NH₄⁺) and its weak conjugate base (NH₃), but their concentrations are not approximately equimolar. If the concentration of conjugate acid and conjugate base are not within a factor of ~10, the buffer will not be able to effectively control pH. This could be an effective buffer if an extra mol of NH₃ was added.
The second combination is equimolar, but HI(aq) is a strong acid, therefore it would not make an effective buffer. This could be an effective buffer if HF and NaF were used instead of HI and NaI.
The third combination is a mixture of a weak base with a strong base, and the resulting solution does not contain an equilmolar combination of a weak conjugate acid and its weak conjugate base, so it is not an effective buffer. This could be an effective buffer if 0.64mols of HCl(aq) was added instead of 0.64mol NaOH(aq), because the resulting solution would contain 0.64mols of HCO₃^-(aq) {a weak conjugate acid} and 0.64mols of CO₃^2-(aq) {its weak conjugate base}.