Review of Chemical Units

The most common units describing elements, chemical constituents, or contaminants in the environment are  

These units are preferred by chemists because they denote quantities that describe chemical reactions. In other words, chemical reactions occur because 1 mol of a given reactant always reacts with a given number of moles of a second reactant, depending on the nature of the reactants. For example, 

1 mol of limestone (CaC03s) reacts with 2 mol of hydrochloric acid in solution to produce 1 mol of CaCl2 in solution, 1 mol of carbon dioxide gas, and 1 mol of water. 

Examples are given below on how one might use this information to prepare a solution of a given concentration. Assume that one needs to prepare 1 mol L-1 NaCl solution. The first thing that is needed is the molecular wei ght (MW) of NaCl, which is the sum of the atomic weights of Na+ (22.99 g mol 1) (g = grams) and Cl- (35.45 g mol-1) (MW NaCl = 58.44 g). Therefore, to make 1 mol L-1 NaCl solution, one needs to dissolve 58.44 g of NaCl in sufficient solute (e.g., distilled water) to make a total volume of 1 L. Hence, 1 mol L-1 NaCl is also 1 eq L 1 NaCl or one mol L-1 Na and 1 mol L-1 Cl. In the preceding statement, the symbols for the elements do not include valence numbers. 

If one prepares a solution of 1 mol L-1 NaCl, it is not clear whether there will be 1 mol L 1 Na+ or 1 mol L 1 Cl-, because the two ions may react with each other or with other chemical species in solution to form additional solution species with different valencies. For example, assuming that the NaCl solution contains also lead (Pb2+), Pb2+ and Cl- would react with each other to form the dissolved chemical species PbCl+, PbCl2, and so on. Chemists distinguish the two situations (free vs. paired solution species) by referring to the total dissolved concentration of an element as formality (F), and to the concentration of certain known dissolved chemical species (e.g., Na+ and Pb2+) as molarity (M) (Table 1.9). Field practitioners of environmental chemistry almost always refer to concentrations of elements because it is total dissolved concen- 

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