Let's check the phone application "buffers" for it's ability to calculate how to prepare a phosphate buffer. Suppose I want 1 liter of a 0.1 M phosphate buffer at a pH of 6.8. Phosphate has a pK2 of 7.2 MW of monosodium phosphate (aka monobasic) is 119.98 MW of disodium phosphate (aka dibasic) is 141.96 You already calculated the fraction of dibasic we need from the H-H equation. In other words you solved for A-/total. That fraction times 0.1 moles is the amount of dibasic, multiply by the MW to get the weight of dibasic we need. Answer will be in grams Calculate to 3 sig figures