Why can’t HCl + NaCl form a buffer? b) Why can’t H2S and Na2S form a buffer? c) Why can HCl + Na2CO3 form a buffer?
The Correct Answer and Explanation is:
Why can’t HCl + NaCl form a buffer?
A buffer solution requires a weak acid and its conjugate base, or a weak base and its conjugate acid. HCl (hydrochloric acid) is a strong acid that dissociates completely in water, meaning it does not have a conjugate base that can help resist pH changes. NaCl (sodium chloride), on the other hand, is a neutral salt and does not have any acidic or basic properties that can assist in stabilizing the pH. Because of this, a solution containing HCl and NaCl cannot act as a buffer because it lacks the necessary components—a weak acid and its conjugate base or a weak base and its conjugate acid—that are required to resist changes in pH.
b) Why can’t H2S and Na2S form a buffer?
Hydrogen sulfide (H2S) is a weak acid, and sodium sulfide (Na2S) is a salt that contains the conjugate base of H2S, which is the sulfide ion (S²⁻). In theory, a buffer could form between H2S and Na2S since they represent a weak acid and its conjugate base. However, the issue arises with the strength of the acid and its conjugate base. H2S is not a very weak acid, and its conjugate base (S²⁻) is very strong, making the buffering range of the system quite narrow. Additionally, the strong conjugate base (S²⁻) tends to be more basic than the acid H2S can effectively neutralize. As a result, H2S and Na2S cannot form an effective buffer in the typical pH range because the pKa of H2S is not ideally matched to the buffering capacity needed for stable pH control.
c) Why can HCl + Na2CO3 form a buffer?
HCl (a strong acid) and Na2CO3 (sodium carbonate) can form a buffer because sodium carbonate contains carbonate ions (CO₃²⁻), which are the conjugate base of carbonic acid (H₂CO₃). While HCl is a strong acid, the carbonate ion can react with excess H⁺ ions from HCl to form bicarbonate (HCO₃⁻), which is a weak acid. This creates a weak acid-conjugate base pair (HCO₃⁻ and CO₃²⁻) that can resist changes in pH. The buffering range of this system is centered around the pKa of bicarbonate, making it effective in buffering pH changes in the acidic range (pH 4.5 to 8.5). Therefore, despite HCl being a strong acid, the presence of Na₂CO₃ creates a buffer system capable of stabilizing pH in this range.
