What amounts of a weak acid and a strong base, when combined, will lead to the formation of a buffer?

Question

BlackTom AI · Accepted Answer

To determine which mixture forms a buffer, we need a solution that contains both a weak acid (HA) and its conjugate base (A-) in appreciable amounts. A buffer typically arises when a weak acid has been partially neutralized by a strong base, leaving both HA and A- present.
Option 1: '1 mol weak acid + 1 mol strong base' — The strong base will fully react with the 1 mole of weak acid: HA + OH- → A- + H2O. This leaves only the conjugate base (A-) in solution and no weak acid, so it cannot act as a buffer. This option fails to provide the necessary HA/A- pair.
Option 2: '1 mol weak acid + 2 mol strong base' — Here, two moles of OH- would neutralize more than the available HA (assuming it's the same HA), driving the reaction past partial neutralization and consuming all HA and creating excess base (A- and OH-). The mixture would not be a typical HA/A- buffer, and the excess base would dominate, so this is not a buffer scenario.
Option 3: '2 mol weak acid + 1 mol strong base' — The strong base neutralizes half of the weak acid: HA + OH- → A- + H2O. After reaction, you have 1 mol HA remaining and 1 mol A- formed, giving a mixture containing both the weak acid and its conjugate base in roughly equal amounts. This is the classic buffer composition for a weak acid–base conjugate pair, providing resistance to pH change.
Option 4: 'You cannot make a buffered solution from a weak acid and a strong base.' — This statement is overly broad and incorrect. While a strong base alone would not yield a buffer with the weak acid, partial neutralization as in option 3 does create a buffer. Buffers do not require only weak base components; they require a weak acid/base conjugate pair in the right proportions. Hence, this option is misleading and incorrect.

What amounts of a weak acid and a strong base, when combined, will lead to the formation of a buffer?Single choice

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