Concentration, the pH Scale, and Indicators

Concentration Units The concentrations used by chemists are measured in molar (M) units. Let us take, for example, a solution of IM NaCl in water. Saying this means that we weighed 1 mole of NaCl (which is the combined atomic weights of Na and Cl, namely 58.44 g/mol) and dissolved it in 1 liter of water. We write concentrations with square brackets, so the concentration of our solution would be written as follows [NaCl] = IM. A liter of water weighs 1,000 g, and since 1 mole of water in gram units is 18.015 (let us round it to 18), then 1 liter of water contains 55.5 moles. Therefore, in a solution of IM NaCl or the same concentration of any other compound, we have for each molecule of the dissolved material 55.5 molecules of water, or better said, there are 110 water molecules for each two molecules of the dissolved material. 

Of course, this process is dynamic, and a molecule that acted as a base in one direction acts as an acid in the reverse direction and vice versa. So the process goes to and fro, and there is a dynamic equilibrium between the forward and the reverse reactions. Equilibrium is another important issue in chemistry (see Zumdahl and Zumdahl, 2010). Although the equilibrium is dynamic (both processes occur all the time), the concentrations of the various species do not change with time. 

SCHEME 8.R.3 Using the curved arrow method to describe the reaction that forms H30 and OH in water. As indicated by the two arrows at the bottom of the drawing, the process is reversible and goes in both directions, what chemists call a dynamic equilibrium. The longer backwards arrow indicates that, at any given time, the concentration of discrete water molecules is larger than that of the ions. 

FIGURE 8.R.1 The molecular forms and colors of phenolphthalein in solutions with pH 7 and pH 7. The scale at the bottom represents different pH values. 

5 Symbolic Representations of Chemical Reactions Using Curved Arrows 

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