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a 150 202.2

b 074 2.33

a 150 202.2

b 074 2.33

Mass concentration. Generally, two methods are used to express mass concentration: mass of solute per unit volume of the mixture (m/v basis- and mass of the solute per unit mass of the mixture (m/m basis). In environmental engineering, the most common expression in the m/v basis is the mg/L. The most common in the m/m basis is the ppm, which means parts per million. In other words, in a ppm, there is one mass of the solute in 106 mass of the mixture.

One ppm for a solute dissolved in water can be shown to be equal to one mg/L. This is shown as follows: 1 ppm = (1 mg)/(106 mg) = (1 mg)/(103 g). The mass density of water at 5°C is 1.0 g/cc. Therefore,

106mg 103g 103cc L

The mass density of water decreases from 1.0 g/cc at 5°C to 0.96 g/cc at 100°C. Thus, for practical purposes,

Molar concentration. In concept, molar concentrations can also be expressed on the m/v basis and the m/m basis; however, the most prevalent practice in environmental engineering is the m/v basis. Molar concentration, then, is the number of moles of the solute per unit volume of the mixture. There are several types of moles: milligram-moles, gram-moles, tonne-moles, and so on corresponding to the unit of mass used. In chemistry, the gram moles is almost exclusively used. When the type of moles is not specified, it is understood to be gram moles. So, normally, molar concentration is expressed in gram moles per liter (gmmols/L).

An important application of molar concentration is in a molar mass balance. For example, in the removal of phosphorus using alum, the following series of reactions occurs:

Al3+ + PO3- — AlPO4(s)i AlPO4(s)i — Al3+ + PO4-po4-+h+ — hpo2-

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