How do you find pH?

Question

A buffer was prepared when someone combined 30 mL of 0.15 M NaF and 30 mL of 0.2 M HF. Ka of HF is 6.6 x 10^-4

What is the pH of this solution?

Answer 1

To find the pH of a buffer, it is easiest to use the Henderson Hasselbalch equation:

pH = pKa + log [salt]/[acid]

The pKa is - log of the Ka for HF. Looking up the Ka for HF I find it to be 7.2x10^-4 and the pKa = 3.14

Next, we find the final [salt] and final [acid] and since they are both diluted by 1/2 (30 ml + 30 ml), we have..

Final [NaF] = 1/2 x 0.15 M = 0.075 M

Final [HF] = 1/2 x 0.2 M = 0.1 M

Now solve the HH equation for pH

pH = 3.14 + log (0.075/0.1) = 3.14 + log 0.75 = 3.14 + (-0.12)

pH = 3.02