How many grams of sodium chloride must dissolve in 750.0 g of water to make a 0.50 molal solution?

Molality is defined as the number of moles of solute in 1 kg of solvent.

molality of solution to be prepared is 0.50 molal

this means that in 1000 g of water there should be 0.50 mol of NaCl

if 1000 g of water should contain - 0.50 mol

then 750.0 g of water requires - 0.50 mol/kg x 0.750 kg = 0.375 mol

mass of NaCl in 0.375 mol - 58.5 g/mol x 0.375 mol = 21.9 g

therefore a mass of 21.9 g of NaCl is required