Which pair of substances will form a buffer solution? HCl; NaCl HNO3; NaNO3 H3PO4, NaH2PO4 H2SO4, CH3COOH NH3, NaOH
The Correct Answer and Explanation is:
Among the pairs listed, H₃PO₄ and NaH₂PO₄ will form a buffer solution.
Explanation:
A buffer solution is formed when a weak acid is combined with its conjugate base (or a weak base with its conjugate acid). This system resists changes in pH when small amounts of acids or bases are added. The effectiveness of a buffer is determined by the presence of these weak acid/base pairs, which can neutralize any added acid or base.
Let’s examine the pairs one by one:
- HCl and NaCl:
HCl is a strong acid and NaCl is the salt of a strong acid and strong base. A strong acid like HCl dissociates completely in solution, and NaCl does not provide a conjugate base to neutralize any added acid. Therefore, this pair cannot form a buffer. - HNO₃ and NaNO₃:
HNO₃ is also a strong acid, and NaNO₃ is a salt formed from a strong acid (HNO₃) and a strong base (NaOH). Similar to HCl, HNO₃ dissociates fully in solution, and there is no weak acid/base pair present. Hence, this pair cannot form a buffer. - H₃PO₄ and NaH₂PO₄:
Phosphoric acid (H₃PO₄) is a weak acid, and NaH₂PO₄ is its sodium salt, which contains the conjugate base H₂PO₄⁻. The presence of both H₃PO₄ and H₂PO₄⁻ in solution makes this a buffer pair. The weak acid (H₃PO₄) can donate protons (H⁺), and the conjugate base (H₂PO₄⁻) can absorb protons, maintaining the pH of the solution. Therefore, this pair can form a buffer. - H₂SO₄ and CH₃COOH:
H₂SO₄ is a strong acid, and CH₃COOH (acetic acid) is a weak acid. Since H₂SO₄ dissociates completely, it does not form a conjugate base that can act as a buffer. Therefore, this pair cannot form a buffer. - NH₃:
NH₃ is a weak base and does not have a conjugate acid (in the context of forming a buffer with only NH₃ as the species). Hence, a buffer cannot be formed with just NH₃.
In conclusion, the only correct buffer pair is H₃PO₄ and NaH₂PO₄.
