If you add 50 mL of 0.20 M NaOH to a 50 mL solution of 0.60 M HF, what is the pH of the resulting buffer

pH = 2.87

Explanation:

When NaOH reacts with HF NaF and H2O are produced:

NaOH + HF → NaF + H2O

That means after the reaction moles of NaF are equal to moles of NaOH added and moles of HF are initial moles of HF - moles of NaOH added:

Moles NaOH = Moles NaF:

0.050L * (0.20mol/L) = 0.01 moles of NaF

Moles HF:

0.050L * (0.60mol/L) = 0.03 moles HF = Initial moles

0.03 moles - 0.01 moles = 0.02 moles HF

To find pH we need to use H-H equation to determine pH of the buffer:

pH = pKa + log [NaF] / [HF]

Where pH is pH of the buffer

pKa is pKa of HF-NaF buffer: 3.17

And [] could be taken as moles of each species.

That is:

pH = 3.17 + log [0.01] / [0.02]