Final answer:
Create a buffer solution with a pH between 4 and 5 using 0.10 M HC₂H₃O₂, the best substance to add is Option 3: 0.050 mol NaC₂H₃O₂, which provides both the weak acid and its conjugate base necessary for a buffer.
Step-by-step explanation:
To determine which substance could be added to 1.0 liter of 0.10 M HC₂H₃O₂ to form a buffer solution with a pH between 4 and 5, we need to understand how a buffer works. A buffer solution is composed of a weak acid and its conjugate base and can resist changes in pH when small amounts of acid or base are added.
HC₂H₃O₂ is a weak acid and its conjugate base is C₂H₃O₂⁻. Adding a strong acid like HCl (Option 1) would not create a buffer but would increase the concentration of H⁺ ions, likely lowering the pH below the desired range. Adding a strong base like NaOH (Option 2) would react with HC₂H₃O₂, but without its conjugate base present in significant amounts, it would not form an effective buffer.
Option 3 involves adding the sodium salt of the conjugate base (NaC₂H₃O₂) to the weak acid, which would produce a buffer solution capable of resisting pH changes. This addition provides both components required for a buffer: the weak acid and its conjugate base.
Option 4 suggests adding both NaOH and NaC₂H₃O₂. The NaOH would neutralize some of the HC₂H₃O₂ to form water and the conjugate base, while the added NaC₂H₃O₂ would contribute to the buffer's capacity. However, adding a stoichiometric amount of NaOH (equal to the moles of HC₂H₃O₂) would lead to a significant shift in the pH that might take it out of the 4-5 range initially, before the rest of the NaOH is added.
Therefore, the best substance to add to 1.0 liter of 0.10 M HC₂H₃O₂ to form a buffer solution with a pH between 4 and 5 is Option 3: 0.050 mol NaC₂H₃O₂.