Question

A sample of 25.0 ml of buffer solution contains 0.100 M fluoride ions and 0.126 M hydrofluoric acid. What is the pH of this buffer solution?

a) pH = 2.87
b) pH = 3.27
c) pH = 3.57
d) pH = 4.17

Answer 1

The pH of the buffer solution is 3.07, which makes it acidic. When 3.00 mL of 0.034 M HCl is added to the buffer solution, the new pH remains the same at 3.17.

Step-by-step explanation:

To determine the pH of the buffer solution, we can use the Henderson-Hasselbalch equation, which is pH = pKa + log([A-]/[HA]). In this case, the weak acid is hydrofluoric acid (HF) and its conjugate base is fluoride ions (F-). The pKa of HF is 3.17, so the equation becomes pH = 3.17 + log([F-]/[HF]). Plugging in the concentrations, we get pH = 3.17 + log(0.100/0.126) = 3.17 + (-0.099) = 3.07.

(b) Since the pH of the buffer solution is 3.07, which is less than 7, the solution is acidic.

(c) When 3.00 mL of 0.034 M HCl is added to the buffer solution, we can determine the new concentrations of HF and F- using the dilution formula: C1V1 = C2V2. Plugging in the values, we get (0.126 M)(25.0 mL) = (0.126 M + x)(25.0 mL + 3.00 mL), where x is the change in concentration of HF. Solving for x, we get x = 0.027 M. Now, we can calculate the new concentrations and apply the Henderson-Hasselbalch equation. The new concentration of HF is 0.126 M - 0.027 M = 0.099 M, and the new concentration of F- is 0.100 M. Plugging these values into the Henderson-Hasselbalch equation, we get pH = 3.17 + log(0.100/0.099) = 3.17 + 0.004 = 3.17.