Question

A 1 liter solution contains 0.383 M hydrofluoric acid and 0.510 M potassium fluoride.

Addition of 0.096 moles of calcium hydroxide will:
(Assume that the volume does not change upon the addition of calcium hydroxide.)
Raise the pH slightly
Lower the pH slightly
Raise the pH by several units
Lower the pH by several units
Not change the pH
Exceed the buffer capacity

Answer 1

Lower the pH slightly

Step-by-step explanation:

The mixture of HF, hydrofluoric acid and KF, potassium fluoride produce a buffer that is defined for the equilibrium:

HF(aq) → H⁺(aq) + F⁻(aq)

The buffer can maintain the pH of a solution despite the addition of strong bases or acids.

The reaction of HF with Ca(OH)2 is:

2HF + Ca(OH)2 → 2H2O + CaF2

That means the calcium hydroxide is decreasing the concentration of HF. Based on the equilibrium, the H+ and F- ions will decrease in order to produce more HF. As H+ is decreasing due the equilibrium and not for the addition of a strong base, the pH is decreasing slightly.