posted by selvi .
The mass of FeSO4.7H2O required to prepare 500mL of 280ppm Fe2+solutin is
280 ppm = 280 g/10^6 g solution. If we call the density of the solution 1.00 g/mL and we ignore the contribution of the mass of the salt to the solution (usually ok since the amount of salt is so small), then 280 ppm = 280 g/10^6 mL which is the same as 0.280 g/10^3 mL = 0.280 g/L. You want 500 mL; you can go from here.