how do you make 400 ml pf 2 Osm solution of Mg CL2?

Answer 1

m is molality and M is molarity; however, I understand Osm is the osmolarity of the solution. I will answer on that assumption.
MgCl2 has a van't Hoff factor of 3; i.e., MgCl2 ionizes in solution to three particles.
How many OSm do you want? That's 0.4 x 2/3 = ?
grams = ? x molar mass MgCl2.