The formula for molality is
molality =moles of solute/kg of solvent
We need to assume something since we are only given %. We can assume 100 g of solution.
Solute: NaCl (what gets dissolved)
Solvent: water (what the solute dissolves in)
Mass of NaCl = 100g(.205)= 20.5g NaCl
Mass of water = 100-20.5g = 79.5 g water(1kg/1000g) = .0795kg
Moles of NaCl = 20.5g/58.44g/mol =0.351mol
molality = 0.351mol/.0795kg= 4.41m
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