The given buffer solution is 0.020 M HF and 0.040 M NaF.
Given the [tex] K_{a} [/tex] of HF is [tex] 3.5 * 10^{-4} [/tex]
Finding out [tex] pK_{a} [/tex]:
[tex] pK_{a} = -logK_{a} [/tex]
= [tex] - log(3.5 * 10^{-4}) [/tex]
= 3.46
We use the Hendersen-Hasselbalch equation to find out the pH of a buffer solution:
[tex] pH = pK_{a} + log\frac{[Base]}{[Acid]} [/tex]
[Base] =[NaF] = 0.040 M
[Acid] = [HF] =0.020 M
Plugging in the values to find out pH,
[tex] pH = 3.46 + log\frac{0.040}{0.020} [/tex]
pH = 3.46 + log(2)
= 3.76
Therefore, pH of the buffer is 3.76
