# A 35.00 mL solution of 0.2500 M HF is titrated with a standardized 0.1532 M solution of NaOH at 25 degrees C. What is the pH of the HF solution before titrant is added? For HF, the Ka = 6.8 x 10–4 . W

Jun 21, 2016

$1.88$

#### Explanation:

A quick way to do this is to use the the expression:

$p H = \frac{1}{2} \left(p {K}_{a} - \log a\right)$

Where $a$ is the concentration of the weak acid.

${K}_{a} = 6.8 \times {10}^{-} 4 \text{ " "mol/l}$

$\therefore p {K}_{a} = - \log \left[6.8 \times {10}^{-} 4\right] = 3.167$

$\therefore p H = \frac{1}{2} \left[3.167 - \log \left(0.25\right)\right]$

$p H = \frac{1}{2} \left[3.167 - \left(- 0.6\right)\right] = 1.88$