posted by Cooper .
What mass of sodium benzoate should you add to 150.00 ml of a 0.15M benzoic acid solution to obtain a buffer with a pH of 4.25?
Use the Henderson-Hasselbalch equation. I would substitute 150*0.15 = 22.5 millimoles for acid and x for sodium benzoate. Solve for x = sodium benzoate, change to mols, then n = grams/molar mass and solve for grams.