? g of glucose C6H12O6 must be dissolved in 400 g of water to give a solution that is 0.258 m.
molality = mols/kg solvent
and mols = g/molar mass
m = mols/kg solvent
0.258 = mols/0.400 kg solvent.
solve for mols
Then mol = g/molar mass.
You know mols and molar mass, calculate grams.