The answer is: H₃PO₄.
A phosphoric acid is three protic acid, which means that in water release tree protons.
Phosphoric acid ionizes in three steps in water.
First step: H₃PO₄(aq) ⇄ H₂PO₄⁻(aq) + H⁺(aq).
Second step: H₂PO₄⁻(aq)⇄ HPO₄²⁻(aq) + H⁺(aq).
Third step: HPO₄²⁻(aq) ⇄ PO₄³⁻(aq) + H⁺(aq).
Species that are present: H₃PO₄, H₂PO₄⁻, HPO₄²⁻, PO₄³⁻ and H⁺.
A buffer can be defined as a substance that prevents the pH of a solution from changing by either releasing or absorbing H⁺ in a solution.
Buffer is a solution that can resist pH change upon the addition of an acidic or basic components and it is able to neutralize small amounts of added acid or base, pH of the solution is relatively stable.
All the solutions listed could be used to make a buffer solution with 0.10 M solution of Na2HPO4 except H3PO4.
We must recall that a buffer consists of a solution of a weak acid and its salt or a solution of a weak base and its salt. As such, a string aci and its salt can not constitute a buffer solution.
Having this in mind, all the solutions listed could be used to make a buffer solution with 0.10 M solution of Na2HPO4 except H3PO4.
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