Answer:
38,7mL of HCl 0,370M
Explanation:
The buffering capacity is lost when the pH of the solution is out of pka ± 1.
For the buffer:
HF ⇄ H⁺ + F⁻; pka = 3,20
Moles of both HF and F⁻ are:
HF: 0,140M ₓ 0,125L = 0,0175 moles of HF
Moles of F⁻ are the same.
Using Henderson-Hasselbalch formula:
pH = pka + log₁₀ F⁻ / HF
pH = 3,20 + log F⁻ / HF (1)
The addition of X moles of HCl (H⁺) shift the equilibrium to the left, thus:
HF ⇄ H⁺ + F⁻
0,0175 moles + X 0,0175 moles - X
The buffer lost its capacity when pH = 3,20 - 1 = 2,20. Thus, replacing in (1):
2,20 = 3,20 + log ₁₀0,0175 - X / 0,0175 + X
0,1 = 0,0175 - X / 0,0175 + X
0,00175 + 0,1X = 0,0175 - X
-0,01575 = -1,1X
0,0143 moles = X
You need to add 0,0143 moles of HCl. In volume for a concentration of 0,370M HCl:
0,0143 moles ÷ 0,370M = 0,0387L of HCl 0,370M ≡ 38,7mL of HCl 0,370M
I hope it helps!
