I have a 100mL buffer solution with a pH of 4.00 using 0.100M benzoic acid (pKa=4.20) and 0.180M sodium benzoate. I need to calculate the volume needed of each substance to create this buffer.

 

So using Henderson-Hasslebalch this is what i came up with

 

4.00=4.20 + log [.180M]/[.100M]

-0.2= log[.180M]/[.100M]

I took the neg. log of -.2 and got

.631 = [.180]/[.100M]

 

now im stuck. shouldnt I have a variable somewhere to solve for? Im not sure what i've missed.

You're trying to find out the amount of benzoic acid and sodium benzoate to add. Not using the concentrations of your stock solution. You need to find the relative concentrations you need. Start by solving for the ratio [A-]/[HA]. Once you get that, figure out how to use this ratio to determine the relative abundances you need (remember, that is a ratio of one to the other, not one to the total). Hope that helps!