Molarity and the pH Scale

you have seen, is capable of acting as either a hydrogen ion donor or a hydrogen ion acceptor, that is, as an acid or a base, depending on the prop -erties of the substance with which it reacts. Water can also act as an acid or a base toward itself, although the reaction occurs to only a very small extent. About 1 of every 550,000,000 water molecules is ionized at any given time. 

Pure water contains equal numbers ofhydronium ions and hydroxide ions. 

The self-ionization of water provides the basis for a convenient method for expressing numerically just how acidic or basic any water solution is. You may have seen this quantity in use for a consumer product, for example, on the label of a pH-balanced shampoo. (The term refers to a controlled acidity of the solution, so that it is less likely to damage hair.) To understand what pH is requires understanding how to express the concentration of a solution, which is the quantity of a solute dissolved in a specific quantity of solvent or solution. We might give the concentration of a solution of sodium hydroxide as 4.0 g/L, that is 4.0 g of NaOH per liter of solution. In chemistry, however, because of its relation to quantities of chemicals in reactions (Section 8.2), concentration based on the mole is preferred. 

Concentrations of solutions in chemistry are usually expressed as molarity, which is the number of moles of solute per liter of solution. For example, a 1 molar (1 M) solution contains 1 mol of solute per liter of solution. If the solute is sodium hydroxide, a 1.00 M solution contains 1 mol, or 40.0 g, of NaOH per liter of solution. To find the molarity of a solution requires knowing the mass of dissolved solute, the volume of the solution, and the molar mass of the solute. For example, 4.0 g of NaOH is 0.10 mol (4.0 g/40.0 g/mol). Thus, a solution with 4.0 g of NaOH dissolved in 1 L of solution is a 0.10 M NaOH solution. 

Finding the number of digits to include in the answers to mathematical problems is reviewed in Appendix A. 

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