A solution consists of 20.5 g of NaCl dissolved in 100 g of solution. What is the molality of NaCl in this solution?

• A. 0.205 m
• B. 0.258 m
• C. 3.51 m
• D. 4.41 m

Answer: (D)

Molality is defined as the number of moles of solute per kilogram of solvent. Here, the solution weighs 100 g total with 20.5 g of NaCl as the solute, so the solvent mass is 100 g − 20.5 g = 79.5 g = 0.0795 kg. Moles of NaCl = 20.5 g ÷ 58.44 g/mol ≈ 0.351 mol. Therefore, molality = 0.351 mol ÷ 0.0795 kg ≈ 4.42 m, which rounds to 4.41 m. Dissociation of NaCl doesn’t affect the calculation since molality uses moles of formula units, not the number of particles in solution.