You wish to prepare 100.0 mL of a 800.0 ppm w/v sodium (MW = 22.9897 g/mol) solution. How many grams of sodium dichromate dihydrate (Na2Cr2O7·2H2O, MW = 297.998 g/mol) are needed to prepare this solution?

Answer 1

You want 800 ppm Na w/v which is 800 mg Na/L or 80 mg Na/100 mL.
Convert 80 mg Na to ?mg Na2Cr2O7.2H2O That will be
80 mg x (molar mass Na2Cr2O7.2H2O/2Na) = ? mg Na2Cr2O7.2H2O to weigh out.