Which of these is not a good method for preparing a buffer? (HA is a weak acid)

Question

BlackTom AI · Accepted Answer

Consider the goal: to prepare a buffer, a system that resists pH change by having a weak acid and its conjugate base (or a weak base and its conjugate acid) in solution.
Option 1: Equal volumes of 1.0 M HA (weak acid) and 0.5 M NaOH (strong base). When mixed, OH− from NaOH neutralizes an equivalent amount of HA to form A− and H2O. Since there is more HA (1.0 M) than OH− (0.5 M) on a per-volume basis, you end up with both HA and its conjugate base A− in solution, which constitutes a valid buffer system.
Option 2: Equal volumes of 0.5 M NH3 (weak base) and 0.5 M NH4Cl (its conjugate acid). This pair directly constitutes a classic buffer system (base–conjugate acid pair) and will resist pH changes upon addition of small amounts of acid or base.
Option 3: Equal volumes of 0.5 M HA (weak acid) and 0.5 M NaA (its salt). This is the textbook preparation of a buffering mixture: the weak acid HA and its conjugate base A− from the salt together provide buffering capacity around the acid’s pKa.
Option 4: Equal volumes of 1.0 M HCl (strong acid) and 0.5 M NaA (conjugate base of HA). Here, HCl will react with A− to form HA (A− + H+ → HA). If the volumes are equal, you convert all or most of A− into HA and leave a large excess of strong acid (HCl) unbuffered, so the solution loses buffering capability and becomes strongly acidic. This does not create a buffer and is therefore not a good method.
In summary, options 1–3 describe typical buffer pairings or predictable neutralizations that yield a buffer, whereas option 4 disrupts buffering by overwhelming the conjugate base with a strong acid, making it the poor choice for buffer preparation.

Which of these is not a good method for preparing a buffer? (HA is a weak acid)单项选择题

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