Question

Calculate the pH of a buffer that is 0.020 M HF and 0.040 M NaF. The Ka for HF is 3.5 × 10-4.

Answer 1

The given buffer solution is 0.020 M HF and 0.040 M NaF.

Given the $K_{a}$ of HF is $3.5 * 10^{-4}$

Finding out $pK_{a}$:

$pK_{a} = -logK_{a}$

= $- log(3.5 * 10^{-4})$

= 3.46

We use the Hendersen-Hasselbalch equation to find out the pH of a buffer solution:

$pH = pK_{a} + log\frac{[Base]}{[Acid]}$

[Base] =[NaF] = 0.040 M

[Acid] = [HF] =0.020 M

Plugging in the values to find out pH,

$pH = 3.46 + log\frac{0.040}{0.020}$

pH = 3.46 + log(2)

= 3.76

Therefore, pH of the buffer is 3.76

Answer 2

 pH = pKa + log ([base]/[acid]) = -log (3.5 x 10^-4) + log (0.040/.020) = 3.46 - 0.30 = 3.16