Question

Buffer 1: 0.1M CA mixed with 0.1M CB

Buffer 2: 0.01M CA mixed with 0.01M CB

Buffer 3: 0.1M CA mixed with 0.01 M CB

Buffer 4: 0.01M CA mixed with 0.1 M CB

This question is from Buffer Capacity lab. Please give me a detailed explanation

How would I make comments on the buffer capacity when add HCl and NaOH to buffer solutions which contain different concentration of CA (conjugate acid) and CB?

Answer 1

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The goal of a buffer is to keep the pH of a solution within a narrow range. While the ratio of conjugate base concn./ conjugate acid concn. influences the pH of a solution, the actual concentrations of conjugate base and conjugate acid influence the effectiveness of a buffer.

The more conjugate base conc. and conjugate acid conc. available, the less of an effect addition of a strong acid or base will have on the pH of a system.

Consider the addition of a strong acid such as HCl. Initially, the HCl donates its proton to the weak base (A^-)through the reaction A^- + HCl → HA + Cl^-. This changes the pH by lowering the ratio [A^-]/[HA], But if we keep adding HCl, the weak base A^- will eventually run out. Once the A^- is gone, any additional HCl will donate its proton to water. This will dramatically increase the concentration [H⁺] and so the pH drops.

This is called "breaking the buffer solution", a solution with more weak base, [A^-], has a higher buffer capacity for addition of strong acid.

Similarly, a buffer will break when the amount of strong base like NaOH is added in so largethat it consumes all the weak acid, through the reaction HA + OH^- → A^-+ H₂O. A solution with more weak acid, [HA], has a higher buffer capacity for addition of strong base.

So although the pH of a buffer is determined by only the ratio [conjugate base / conjugate acid ], the ability of the buffer to absorb strong acid or base is determined by the individual concentrations of A- and HA- .