[Histonet] help! buffer calculation

[Neil Fournier]

To make a 1-L 0.2 M phosphate buffer stock my recipe calls for 21.87 g Na2HPO4 (anhydrous dibasic) and 6.35 g NaH2PO4.H2O (monohydrate monobasic) dissolved in 1-L of water.  (fairly standard procedure).   I am sure my undergraduate chemistry professor is turning over in his grave, but I am embarrass to say that I am having difficulty figuring out how  the amount of anhydrous dibasic and monohydrate monobasic was determined. Could someone please show me the calculations. I have played around with Henderson-Hasselbalch equation to no end.

Much obliged

Neil