How many grams of lithium sulfide must be dissolved in 1600.0 g water to make a 2.0 molal solution?

Molality is the number of moles of solutes in 1 kg of solvent.
the molality of solution to be prepared is 2.0 molal. 
therefore 2 moles in 1 kg water.
the mass of Li₂S required is - 46 g/mol x 2.0 mol = 92 g
the mass in 1 kg of solvent is - 92 g
Therefore mass of Li₂S required in 1600.0 g is - 92 g/kg x 1.6 kg = 147.2 g