Which one of the following combinations does not create a buffer?

• A. HC₂H₃O₂ and KC₂H₃O₂
• B. H₂SO₃ and NaHSO₃
• C. H₃PO₄ and NaH₂PO₄
• D. HNO₃ and KNO₃
• E. NH₄Cl and NH₃

Answer: (D)

Verified step by step guidance

1. Understand what a buffer solution is: A buffer solution is a system that resists changes in pH when small amounts of acid or base are added. It typically consists of a weak acid and its conjugate base, or a weak base and its conjugate acid.
2. Analyze each combination to determine if it forms a buffer: A buffer is formed when there is a weak acid and its conjugate base or a weak base and its conjugate acid present in the solution.
3. Examine the combination HC2H3O2 and KC2H3O2: HC2H3O2 is acetic acid (a weak acid) and KC2H3O2 is potassium acetate (the conjugate base of acetic acid). This combination forms a buffer.
4. Examine the combination H2SO3 and NaHSO3: H2SO3 is sulfurous acid (a weak acid) and NaHSO3 is sodium bisulfite (the conjugate base of sulfurous acid). This combination forms a buffer.
5. Examine the combination HNO3 and KNO3: HNO3 is nitric acid (a strong acid) and KNO3 is potassium nitrate (a salt that does not provide a conjugate base of a weak acid). This combination does not form a buffer because it involves a strong acid and its salt, which does not resist changes in pH.