Question

What is the molality, m, of a solution made by adding 4.18 g of solid naphthalene, C10H8, to 32.01 g of liquid benzene, C6H6? The molar mass of naphthalene = 128.2 g/mol and benzene = 78.1 g/mol.

Answer 1

Mass of naphthalene = 4.18 g

Molecular Mass of naphthalene = 128.2 g/mol

So number of Moles naphthalene = Mass / Molecular Mass = 4.18 / 128.2 = 0.0326 moles

Mass of Benzene = 32.01 g = 0.03201 Kg

Molality is a property of a solution and is defined as the number of moles of solute per kilogram of solvent. The SI unit for molality is mol/kg

Molality = Moles of Solute / Mass of Solvent in Kg

=> m = 0.0326 moles / 0.03201 Kg = 1.01 moles / Kg

So molality = 1.01 moles / Kg

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