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A solution of NaCl in water has a concentration of 20.5% by mass. What is the molal concentration of the solution?

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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