Question

calculate the mass of sodium benzoate that must be added to 1 litre of 0.4M benzoic acid with Ka=6.4 multiplied by 0.00004 solution to a buffer at pH 4.5
DrBob222
You need to read what you posted and clarify it. It makes no sense as is. For example Ka = WHAT?
At any rate, clean it up and use the Henderson-Hasselbalch equation.