What mass of MgCl2 should be dissolved in 75.0 g H2O to produce a solution that is 38.5% solute by mass?
% m/m (or w/w) = grams solute/100 g solution or
% MgCl2 = [(grams MgCl2/(75 + g MgCl2)]*100 = 38.5. Solve for g MgCl2. Post your work if you get stuck.