calculate the mass of Cuso4.5H2O required to prepare 500 cm3 of a 0.400 m/dm-3 solution
Assuming you meant 0.400g/dm^3, that translates to 0.0004 g/cm^3
So, 500cm^3 of that means 0.2 g
Or, in one step, that's
500 cm^3 * (1dm/10cm)^3 * 0.4 g/dm^3
I think 0.400 m/dm3 meant 0.400 mols/dm3 = 0.400 M.