1. Understand Molality

Molality (m) is a measure of concentration defined as the number of moles of solute per kilogram of solvent.

2. Calculate the Moles of Lithium Sulfide

* You want a 2 molal solution, meaning 2 moles of lithium sulfide (Li₂S) per kilogram of water.

* Since you have 1600 g of water, convert that to kilograms: 1600 g / 1000 g/kg = 1.6 kg of water

* Calculate the moles of Li₂S needed: 2 mol/kg * 1.6 kg = 3.2 moles of Li₂S

3. Calculate the Mass of Lithium Sulfide

* Find the molar mass of Li₂S:

* Li: 6.941 g/mol (x 2 = 13.882 g/mol)

* S: 32.065 g/mol

* Total: 13.882 g/mol + 32.065 g/mol = 45.947 g/mol

* Calculate the mass of Li₂S needed: 3.2 moles * 45.947 g/mol = 146.63 g

Answer: You need to dissolve 146.63 grams of lithium sulfide in 1600 grams of water to make a 2 molal solution.