Final answer:
Na₂HPO₄ will form a buffer with HPO₄²⁻ because it will provide the corresponding conjugate acid when dissolved in water, which is necessary for a buffer solution. So, the correct option is C.
Step-by-step explanation:
A buffer solution contains a mixture of a weak acid and its conjugate base, or a weak base and its conjugate acid. This allows the buffer to resist changes in pH when small amounts of acid or base are added. HPO42- is the conjugate base of the weak acid H2PO4-. To form a buffer with HPO42-, we need to add either the weak acid H2PO4- itself, or a salt containing the H2PO4- ion.
Looking at the options provided:
- NaOH is a strong base and does not provide the necessary conjugate acid.
- H2SO4 is a strong acid and would react completely with HPO42- rather than form a buffer.
- Na2HPO4 contains the HPO42- ion and would provide the corresponding conjugate acid H2PO4- when dissolved in water, making it the correct choice.
- KOH is also a strong base and, like NaOH, does not provide the conjugate acid.
Therefore, the correct answer is (c) Na2HPO4, as it will combine with HPO42- to create a buffer solution.