ravenhawk_49417

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.

I am creating an unknown riboflavin solution. I have a mulit-vitamin tablet weighing 0.849g. The bottle says there is 10g of riboflavin in each tablet. The tablet was crushed up and I took 0.201g of the powder and mixed it in 250mL of distilled water. I need to calculate the mass of riboflavin in the entire unknown solution and I am not sure how to set it up. At first I thought you take the 0.849g tablet/10g riboflaven = X/.201g vitamin sample. then X would equal 0.422g riboflavin but where does the 250ml of water in the solution come into play?