Answer to Question #338908 in General Chemistry for maria

Question

1.    What concentration of NaF do you need to add to a 0.15 M HF solution to get a buffer solution pH of 9.5? The Ka for HF is 7.4x10^-4.

Answer 1

pKa = - log (7.4×10^-4) = 3.13

pH = pKa + log ([NaF]/[HF]) 

9.50 = 3.13 + log ([NaF]/[HF]) 

log ([NaF]/[HF]) = 6.37

[NaF] / 0.15 = 10^6.37

[NaF] = 10^6.37/0.15

Most likely the problem has an incorrect pH value, usually it is not alkaline.