How many grams pure K 2 S 2 O 5 do you need to add to a 2500 L tank to achieve a 0.83 mg/l molecular SO2? Ph 3.4, Free SO2 17mg/l, Total SO2 85 mg/l Need to determine how much free SO2 needed for the molecular SO2 for the equation K 2 S 2 O 5 (g) to add = VxD/A V=volume D=dose mg/l SO2 to add A=SO2 activity as a decimal fraction