A buffer solution may result if K3PO4 is mixed with

Question

BlackTom AI · Accepted Answer

Question restatement: A buffer solution may result if K3PO4 is mixed with

Options are:
- K2HPO4.
- either HCl or K2HPO4.
- NaOH.
- HCl.
- either K2HPO4 or NaOH.

Option-by-option analysis:
Option 1: K2HPO4.
This salt provides the dihydrogen phosphate conjugate base HPO4^2−. On its own, it does not supply the complementary conjugate acid (H2PO4−) to form a classic buffer pair. Therefore, a mixture containing only K3PO4 and K2HPO4 would predominantly consist of two base species (PO4^3− and HPO4^2−) and lacks the necessary acid–base pair to resist pH changes over a range. In short, this by itself does not guarantee a buffering system.

Option 2: either HCl or K2HPO4.
This option suggests two possibilities:
- K3PO4 with HCl: Adding strong acid to a basic phosphate solution can convert some PO4^3− into HPO4^2−, generating a conjugate-base pair (HPO4^2−/PO4^3−) that buffers around the pKa for the HPO4^2−/PO4^3− equilibrium, especially near pH ~12.4. The presence of both species provides capacity to absorb added acid or base at that region.
- K3PO4 with K2HPO4: As discussed in Option 1, this creates a mixture of PO4^3− and HPO4^2−, which is a valid buffering pair around that same region. So this indeed yields a buffer.
Thus, this option correctly identifies two scenarios that produce a buffer.

Option 3: NaOH.
Introducing a strong base (NaOH) to a phosphate system would push the equilibrium toward higher concentrations of PO4^3− and OH−, and would not create the necessary conjugate-acid/conjugate-base pair in the pH region typical for buffering here. Rather than forming a buffer, you’d likely obtain a more basic solution or shift equilibria without a stable buffering pair. Therefore, this choice does not describe a buffering mixture.

Option 4: HCl.
Adding just HCl to K3PO4 without considering stoichiometry would largely neutralize PO4^3− to form HPO4^2−, but if all strong acid were consumed and the system moved toward H2PO4−, you would depart from a simple buffer in the phosphate system at that region and could reach a different speciation. The key point is that HCl can participate in forming the HPO4^2−/PO4^3− buffer pair when present with K3PO4, but as a standalone statement, saying only HCl guarantees a buffer is incomplete. In the context of the given choices, HCl is a valid way to establish a buffer when paired with K3PO4, but the option “HCl” alone is less explicit about the necessary partner species; the exam’s intended correct phrasing includes HCl as one of the buffering routes because it can generate the needed pair.

Option 5: either K2HPO4 or NaOH.
This option claims buffering occurs with K2HPO4 (which is questionable by itself, as discussed in Option 1) or with NaOH (which would not yield a buffer). Since one branch (NaOH) is incorrect, this broad statement is not accurate.

Summary of reasoning:
- The essential buffering pair in this phosphate system is PO4^3−/HPO4^2−, which buffers around pH ~12.4. Any mixture that includes both species (either by starting with K3PO4 and adding HCl to generate HPO4^2−, or by directly using a mixture of PO4^3− and HPO4^2− from K3PO4 and K2HPO4) forms a buffer.
- Choices that do not establish that conjugate-base/conjugate-acid pair (or do so only ambiguously) fail to guarantee buffering behavior in the intended context.
- The two viable routes described by the question are: mixing K3PO4 with HCl (partial neutralization yielding HPO4^2−/PO4^3−) and mixing K3PO4 with K2HPO4 (conjugate-base pair HPO4^2−/PO4^3− already present).

Therefore, the statement 'either HCl or K2HPO4.' correctly captures the two ways a buffer can form with K3PO4 in this context. The other options either omit a necessary component of the buffer pair or describe conditions that do not establish a phosphate buffer system in the expected pH range.

A buffer solution may result if K3PO4 is mixed with 单项选择题

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