A 29.2 mL sample of a 0.458 M aqueous hydrofluoric acid solution is titrated with a 0.337 M aqueous sodium hydroxide solution. What is the pH at the start of the titration, before any sodium hydroxide has been added

3.51

Explanation:

Before any sodium hydroxide has been added, the pH is that of the aqueous hydrofluoric acid solution.

HF is a weak acid that dissociates according to the following equation.

HF (aq) ⇄ H⁺ (aq) + F⁻ (aq) Ka = 6.76 * 10⁻⁴

We can find [H⁺] using the following expression.

[H⁺] = √ (Ca * Ka)

where

Ca: concentration of the acid

Ka: acid dissociation constant

[H⁺] = √ (Ca * Ka)

[H⁺] = √ (0.458 * 6.76 * 10⁻⁴)

[H⁺] = 3.10 * 10⁻⁴ M

The pH is:

pH = - log [H⁺]

pH = - log 3.10 * 10⁻⁴ = 3.51