Question

DIRECT METHOD EXAMPLE: A buffer was created by mixing 10 mL of a 0.10 M hydrofluoric acid solution with 25 mL of a 0.20 M sodium fluoride solution. What is this solution’s pH? Do you expect it to be a ‘good’ buffer with a high buffer capacity or a ‘poor’ buffer with low capacity? The Ka of hydrofluoric acid is: 7.2 x 10-4. Show work, thank you.

Answer 1

first calculate moles of both reactants:
moles of NaF = 0.025 * 0.20 = 0.005 moles
moles of HF = 0.010 * 0.1 = 0.001 moles

pH = pKa + log[A^-]/[HA]
pKa = -log(7.2x10^-4) = 3.14
pH = 3.14 + log(0.005/0.001)
pH = 3.84

The goal of a buffer is to keep the pH of a solution within a narrow range. While the ratio of [A^-]/[HA] influences the pH of a solution, the actual concentrations of A^- and HA influence the effectiveness of a buffer.

The more A^- and HA molecules 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 as long as there is still a lot of A^- present, the change in pH will be small. 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 (HCl + H₂O → H₃O⁺ + Cl^-). This will dramatically increase the concentration [H⁺] and so the pH drops.

We call this "breaking the buffer solution", and we call the amount of acid a buffer can absorb before it breaks the "buffer capacity for addition of strong acid". 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 added is so large 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 [A^-]/[HA], the ability of the buffer to absorb strong acid or base is determined by the individual concentrations of [A^-] and [HA].

So basing on this we can say that it has a low capacity (low HA).

Hope this helps