Calculate the volume of .300 M NaOH needed to prepare a pH=3.50 buffer with 250ml of .075M HF solution. pKa (HF) = 3.15

Answer 1

You have 250 mL x 0.075M = 18.75 mmoles HF.
........HF + NaOH ==> NaF + H2O
initial.18.75..0.......0......0
add.............x................
change..-x.....-x......+x.....+x
equil..18.75....0........x.....x

pH = pKa + log [(base)/(acid)]
3.50 = 3.15 + log(x/18.75-x)
Solve for x which gives millimoles, then
mmoles = mL x M
You know mmoles and M, solve for mL of the 0.3M NaOH to add.