A solution is a homogeneous mixture of two or more components. The component in the largest amount is called the solvent and the other components are called the solute(s). We need to have methods of stating the concentrations of all components. One method of specifying concentrations is molarity, which we have already used in stoichiometry problems. Molarity, M, is the number of moles of solute per liter of solution and is a very useful method for some applications. However, for applications in which we need to know the number of molecules of solute per molecule of solvent, it is not very useful. Molality, m, is the number of moles of solute per kilogram of solvent and this unit does allow us to determine a number ratio. The mole fraction, XA, is the number of moles of one solution component divided by the total number of moles of all components. Obviously the sum of all of the mole fractions of a solution is equal to one. The weight percent is exactly what it says it is: the mass of one solution component divided by the total solution mass times 100 to convert a fraction into a percent. The last method discussed is the parts per million (ppm) designation, which refers to relative amounts by weight. A solution concentration of 1.0 ppm would have 1.0 g of solute per 1.0 x 106 g of solvent. Because water has a density of 1.0 g/ml at 25 oC, ppm and mg/liter are approximately equivalent when applied to water solutions.
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