Question

Buffer capacity is a measure of a buffer solution\'s resistance to changes in pH as strong acid or base is added. Suppose that you have 145 mL of a buffer that is 0.160 M in both hydrofluoric acid (HF) and its conjugate base (F–). Calculate the maximum volume of 0.160 M HCl that can be added to the buffer before its buffering capacity is lost.

Answer 1

pka of HF = 3.17

pH of acidic buffer = pka + log(salt/acid)

let me take initial pH of buffer = 3.17 at this buffer capacity is maximum

so that ,

No of mole of buffer taken = 0.145*0.16 = 0.0232 mol

3.17 = 3.17 + log(x/(0.0232-x))

x = 0.0116 mol

No of mole of congugate base = 0.0116 mol

No of mol of HF = 0.0232 - 0.0116 = 0.0116 mol

now the maximum buffer capacity = ka+1 or ka-1

addition of HCl to the acidic buffer pH DECREASES

2.17 = 3.17+log((0.0116 - x)/(0.0116+x))

x = 0.0095 mol

volume of HCl needed = 0.0095 / 0.16 = 0.06 L = 60 ml