Interconversion of Composition Measures

Given the solution composition in mass percent, it is easy to compute the mole fractions of solute and solvent and the molal concentration of solute.

Example 1

What are the mole fractions and molal concentrations in a 10% by weight solution of CH₃OH in water? We first compute the number of moles of CH₃OH and water:

The total number of moles is 5.33 and the mole fractions are:

The molal concentrations of CH₃OH is computed from the number of moles and the mass of H₂O.

Molar concentrations involve the volume of the solution and to get this from the mass, we need the density.

Example 2

Given that the density of 10.0% aqueous NaCl is 1.071 g/cm³, what is the molar concentrations of Na⁺ and Cl^-?

Two approaches are possible. (1) We know how many moles of NaCl are contained in 100 g of solution. Thus we have

(2) One liter of solution weighs 1071 g and contains 107.1 g NaCl/L; converting to moles, we have [NaCl] = [Na⁺] = [Cl^-] = 1.83 mol/