how would you prepare 500 ml of .2M solution starting with solid NaH2PO4*H20?

1 answer

How many moles do you want? That's M x L = 0.2 x 0.500 =?
Then mols = grams/molar mass. You know mols and molar mass, solve for grams.
Weigh out that many grams, place in a 500 mL volumetric flask, swirl until all of the solid is dissolved, make to the mark with distilled water, shake thoroughly.