CONCENTRATION UNITS AND THE PH SCALE

Two units are routinely used to express concentrations of acids and bases:

  • • Molarity, M
  • • pH scale

Molarity is defined as the number of moles of H+ ion for acids or of OH- ion for bases per liter of solution. Concentration is symbolized by the use of brackets; that is, [ ] means moles per liter of the bracketed quantity.

The pH, representing the power of hydrogen ions, can be calculated from molarity. pH is useful for solutions whose acidic concentrations are less than 1.0 M, for example, 3.5 * 10-3 M.

Equation 1.22 may also be written as follows:

Although not an absolute restriction, the pH scale conventionally runs from 0 to 14, because of the autoionization of water. Recall that Kw for water is 1.0 * 10-14 at 25 °C, since Kw = [H+][OH-] = 1.0 * 10-14.

The pH of some common substances

Figure 1.3. The pH of some common substances

The pOH may be analogously computed for basic solutions:

In addition, since

it is always possible to find pOH from pH, or vice versa.

For the more commonly used pH, the acid range of solutions is 0 to 7, while the basic or alkaline range is 7 to 14. A pH of 7.0 represents a neutral solution such as distilled water. The pH values of common substances are shown in Figure 1.3.

Properties of acids include the following:

  • • They taste sour.
  • • They neutralize bases to produce water and a salt.
  • • They react with many metals to yield H2 gas.
  • • They are strong dehydrating agents.

Properties of bases include the following:

  • • They taste bitter.
  • • They feel slippery.
  • • They react with fats and oil.
  • • They neutralize acids to produce water and a salt.
 
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