A hydrofluoric acid buffer solution has the following concentrations:

[HF]=0.43M
[F-]=0.45M
If 40 mL of 0.1M NaOH is added to 500 mL of the buffer, what is the resultant pH?

millimols HF = mL x M = 500 mL x 0.43M = 215

mmols F^- = 500 mL x 0.45M = 225
40 mL x 0.1M NaOH = 4 millimols.

...........HF + OH^- ==> F^- H2O
initial...215....0......225......
add.............4.0............
change...-4.....-4........+4.........
equil....211......0.....229

Substitute the equil line from the ICE chart into the HH equation and solve for pH.