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4. Salts are made up of a cation (other than H⁺) and an anion (other than OH^- or oxide, O^2-). The formula of the salt indicates the acid and base that could react to prepare the salt. The cation (positive ion) is derived from the base; the anion (negative ion) is derived from the acid.

Figure 1. Formation of salt from acid and base.

5. A strong electrolyte is an electrolyte that exists in solution almost entirely as ions. Hydrochloric acid, HCl, is a strong electrolyte. A weak electrolyte is an electrolyte that dissolves in water to form an equilibrium between a molecular substance and a relatively small quantity of ions. Acetic acid and aqueous ammonia are examples of weak electrolytes.

6. In aqueous solution, H⁺ (that is, H3O⁺) is the strongest stable acid and OH^- is the strongest stable base.

7. pH is the negative logarithm of the molar hydrogen ion concentration; pOH is the negative logarithm of the molar hydroxide ion concentration. Mathematically,

             pH  =  -log [H⁺]    and    pOH  =  -log [OH^-]

8. pH + pOH = 14.0 (at 25 deg.C)

9. The relative acidity (or basicity) of a solution can be experimentally determined with an acid-base indicator. Indicators are either weak acids or weak bases that change colors over a short pH range. That point in a titration when a suitably-chosen indicator changes color is called the end point. Some typical acid-base indicators, their acid and base colors, and the pH range for the color change are given in Figure 2.

Indicator	Acid color	Base color	pH range
Bromophenol blue	Yellow	Blue-violet	3.0-4.5
Methyl orange	Red	Yellow	3.1-4.4
Universal indicator	Red	Blue-violet	4.0-10.0
Litmus	Red	Blue	4.7-8.2
Methyl red	Red	Yellow	4.8-6.2
Brothymol blue	Yellow	Blue	6.0-7.6
Phenol red	Yellow	Red	6.8-8.4
Phenolphthalein	Colorless	Red	8.2-10.0

Figure 2. Indicator Colors and pH Range.

10. A pH meter can give quantitative pH values to several decimal places. A specially-designed electrode is dipped into the solution to be tested. An electrical potential is generated between the electrodes which depends on the pH. The electrical potential, in volts, is read on a meter calibrated directly in pH or digitally displayed.

Acids and Bases
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