Answer
$pH = 3.46$
Work Step by Step
1. Draw the ICE table (It is in the end of this answer)
2. Calculate 'x' using the $K_a$ expression.
$ 3.5\times 10^{- 4} = \frac{[F^-][H_3O^+]}{[HF]}$
$ 3.5\times 10^{- 4} = \frac{( 0.15 + x )* x}{ 0.15 - x}$
Considering 'x' has a very small value.
$ 3.5\times 10^{- 4} = \frac{ 0.15 * x}{ 0.15}$
$ 3.5\times 10^{- 4} = 1x$
$x = 3.5\times 10^{- 4}$
Percent dissociation: $\frac{ 3.5\times 10^{- 4}}{ 0.15} \times 100\% = 0.2333\%$
x = $[H_3O^+]$
3. Calculate the pH Value
$pH = -log[H_3O^+]$
$pH = -log( 3.5 \times 10^{- 4})$
$pH = 3.46$
