If a solution of hf (ka=6.8×10−4) has a ph of 2.90, calculate the concentration of hydrofluoric acid.

0.0023

Explanation:

1) Balanced equilibrium equation:

    HF ⇆ H⁺ + F⁻

2) Equilibrium constant of the acid, Ka:

        Ka = [H⁺] [F⁻] / [HF]

       Hence:

       Ka = [H⁺] [H⁺] / [HF] = [H⁺]² / [HF]

3) From the pH value you can calculate [H⁺],  using the definition:

4) Now you can substitute [H⁺] and the value of Ka in the equation for Ka:

   Ka = [H⁺]² / [HF] ⇒ [HF] = [H⁺]² / Ka = (0.00126)² / (6.8×10⁻⁴) = 0.00233.

Since the value of Ka has two significant figures, you must report the answer with two significan figures, i.e. 0.0023

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