The molar weight of MgSO4 is 120.366gmol-1. What is the mass/volume percentage of a 2.5M water solution of MgSO4?
2.5M means 2.5 mols/1000 mL solution.
2.5 mols x 120.366 = estimated 300 g and that is in 1000 mL. So 30g must be in 100 mL and that makes it an estimated 30%