What Mass of Sodium Fluoride Must Be Added to 250

Multiple Choice

What mass of sodium fluoride must be added to 250.mL of a 0.100 M HF solution to give a buffer solution having a pH of 3.50? [K_a(HF) = 7.1 × 10^-4]

A) 0.49 g
B) 1.5 g
C) 3.4 g
D) 2.3 g
E) 0.75 g