2. A 100.00 mL volumetric flask is loaded with 1.60 grams of NaF. To this was added 30.0 mL of a 4.00 M solution of HF. When this flask is filled to the mark and shaken to ensure complete mixing, what is the pH of the final solution

pH of the final solution = 3.8

Explanation:

Concentration of NaF = molar

                                    = 0.3 molar

NaF → Na⁺ +    F⁻

HF   ⇆ H⁺    +    F⁻

         according to common ion effect dissociation of weak acid decreases.

        Ka =

⇒    [H⁺]  = ...............(1)

{Ka of HF = 3.5 x 10⁻⁴} & Concentration of HF = 30 x 4 x 10⁻³ = 0.12 molar

from equation 1

⇒ [H⁺]  =     [Concentration of F⁻ ≡ Concentration of NaF]

⇒ [H⁺] = 0.00014

⇒pH = - log 0.00014 = 3.85